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The Genus Xiphophorus in Mexico and Central America
Abstract
The genus Xiphophorus is found from northeastern Mexico (Coahuila) for about 2200 Km as far as Honduras. There are 26 species, of which 21 occupy headwaters on the eastern slope of the Sierra Madre Oriental and continuing Cordillera to the southeast. Virtually all the species in the headwaters occupy limited ranges, often in rivers traversing karst country that are separated from lowland streams by underground passages. Only the three forms in the coastal plain are more widely distributed. Nineteen taxa occur within 400 Km of the Mexican Trans Volcanic Belt, suggesting that the genus may have evolved in this region. In many localities two species are sympatric, but natural hybrids are only known from three or four sites. Four monophyletic groups have been identified: the northern platyfish and the northern swordtail groups, north of the Mexican Trans Volcanic Axis, and to the south the helleri and the clemenciae swordtail groups. The status of the three southern platyfish is still not resolved and the phylogenetic relationship of the different groups to each other is still not fully understood.
... To determine whether the variation in specific gonopodial traits is correlated with habitat type, that is, sites with different water flow regimes such as ponds versus flowing rivers, we used habitat data descriptions from all existing studies where water flow has been characterized for Xiphophorus habitats (Rosen 1960;Rauchenberger et al. 1990;Meyer and Schartl 2003;Kallman et al. 2004;Kallman and Kazianis 2006;Jones et al. 2012), as well as from unpublished data collected and verified over 35 years of regular field studies (M. Schartl, unpublished data). ...
... We asked whether or not those pairs that are sympatric in nature have more pronounced differences in gonopodial structure than pairs that are allopatric in nature. We utilize extensive interspecific hybridization records (both under laboratory conditions, Schartl et al. unpublished, and naturally hybridizing species, summarized in Kallman and Kazianis (2006)], as well as species geographical distribution information including sympatric and allopatric data (Tables S3, S4). We investigated sympatry and hybridization using, as outlined above, partial Mantel tests. ...
... These tests were implemented because sympatry and hybridization events can be expressed only as a property of species pairs and we could therefore not use the phylogenetic generalized least-squares method to test for difference in the five morphometric traits. Specifically, we tested for the correlation between the matrix of pairwise morphometric distances (after allometric correction) and a binary matrix reflecting, respectively, if each pair of species lived in sympatry or not, if each pair of species hybridized under laboratory conditions, and if each pair of species hybridized under both laboratory and natural conditions (see Tables S3 and S4: data compiled from Rosen 1979;Meyer 1983;Kallman et al. 2004;Kallman and Kazianis 2006; M. Schartl pers obs.). As above, the matrix of patristic distances obtained from the phylogeny of Jones et al. (2013) was used to account for phylogenetic nonindependence in all tests. ...
Internally fertilizing animals show a remarkable diversity in male genital morphology that is associated with sexual selection, and these traits are thought to be evolving particularly rapidly. Male fish in some internally fertilizing species have “gonopodia,” highly modified anal fins that are putatively important for sexual selection. However, our understanding of the evolution of genital diversity remains incomplete. Contrary to the prediction that male genital traits evolve more rapidly than other traits, here we show that gonopodial traits and other nongonopodial traits exhibit similar evolutionary rates of trait change and also follow similar evolutionary models in an iconic genus of poeciliid fish (Xiphophorus spp.). Furthermore, we find that both mating and nonmating natural selection mechanisms are unlikely to be driving the diverse Xiphophorus gonopodial morphology. Putative holdfast features of the male genital organ do not appear to be influenced by water flow, a candidate selective force in aquatic habitats. Additionally, interspecific divergence in gonopodial morphology is not significantly higher between sympatric species, than between allopatric species, suggesting that male genitals have not undergone reproductive character displacement. Slower rates of evolution in gonopodial traits compared with a subset of putatively sexually selected nongenital traits suggest that different selection mechanisms may be acting on the different trait types. Further investigations of this elaborate trait are imperative to determine whether it is ultimately an important driver of speciation.
... Xiphophorus continens is closely related to X. pygmaeus [39,44], and like all other northern swordtails, it occurs in the Rio Panuco drainage [59]. Xiphophorus kallmani is endemic to the Laguna de Catemaco, which is within the geographic distribution of X. hellerii [60]. Our analyses were missing three species of platyfishes. ...
... Xiphophorus gordoni and X. meyeri are closely related to X. couchianus comprising a single, monophyletic clade. All three species occur in tributaries of the Rio Grande basin and have restricted ranges in an arid region of northern Mexico that is geographically removed from the remainder of the genus [60]. Accordingly, the three species may inhabit similar environments and exhibit similar niches to one another, but as a group are likely divergent from other platyfishes. ...
... Accordingly, the three species may inhabit similar environments and exhibit similar niches to one another, but as a group are likely divergent from other platyfishes. Finally, X. andersi is a more basal platyfish [39,44] that occurs south of the TMVB, but inhabits a high-elevation, headwater habitat that is distinct from its closest relatives X. maculatus and X. xiphidium [60]. (2) Differences in the degree of niche conservatism between the swordtails and platyfishes could be a consequence of the environmental variables included in this study. ...
Background: Ecological factors often have a strong impact on spatiotemporal patterns of biodiversity. The integration of spatial ecology and phylogenetics allows for rigorous tests of whether speciation is associated with niche conservatism (constraints on ecological divergence) or niche divergence. We address this question in a genus of livebearing fishes for which the role of sexual selection in speciation has long been studied, but in which the potential role of ecological divergence during speciation has not been tested.
Results: By combining reconstruction of ancestral climate tolerances and disparity indices, we show that the earliest evolutionary split in Xiphophorus was associated with significant divergence for temperature variables. Niche evolution and present day niches were most closely associated with each species’ geographic distribution relative to a biogeographic barrier, the Trans-Mexican Volcanic Belt. Tests for similarity of the environmental backgrounds of closely related species suggested that the relative importance of niche conservatism and divergence during speciation varied among the primary clades of Xiphophorus. Closely related species in the two swordtail clades exhibited higher levels of niche overlap than expected given environmental background similarity indicative of niche conservatism. In contrast, almost all species of platyfish had significantly divergent niches compared to environmental backgrounds, which is indicative of niche divergence.
Conclusion: The results suggest that the relative importance of niche conservatism and divergence differed among the clades of Xiphophorus and that traits associated with niche evolution may be more evolutionarily labile in the platyfishes. Our results ultimately suggest that the taxonomic scale of tests for conservatism and divergence could greatly influence inferences of their relative importance in the speciation process.
... Xiphophorus species are found in a very broad geographical range, exceeding 2,200 km, from northern Mexico and south to Honduras. This extensive range following the Sierra Madres uplift harbors many different environments at a wide range of altitudes (e.g., sea level to 1,200 m, Fig. 1) [1]. Schartl et al. (2013) recently reported the whole-genome sequencing and assembly of a platyfish, Xiphophorus maculatus Jp 163 A, thus detailing the first genome architecture from a live-bearing Poeciliid fish [2]. ...
... In addition to the whole genome, transcriptomes of X. maculatus have been sequenced, de novo assembled from RNAseq data [4,5] and annotated using homologous coding sequences from related species (Ensembl genebuild pipeline). The availability of the X. maculatus reference genome and transcriptome assemblies have greatly accelerated the identification of differences that are coincidental with speciation, the evolution of genetic incompatibility, and the genetics underlying pigment pattern expression, and sex determination in Xiphophorus [1,[5][6][7][8][9][10]. ...
... Due to this scientific history, and an ever increasing use of Xiphophorus in contemporary experimental biology, the Xiphophorus Genetic Stock Center (XGSC) was first established in the 1930's and has remained in continuous operation as one of the oldest live animal resource centers worldwide. Twenty-four Xiphophorus species and 55 pedigreed lines are maintained in the XGSC and fish lines that have been sequenced for this study are available for research upon request [1,9]. ...
Xiphophorus fishes are represented by 26 live-bearing species of tropical fish that express many attributes (e.g., viviparity, genetic and phenotypic variation, ecological adaptation, varied sexual developmental mechanisms, ability to produce fertile interspecies hybrids) that have made attractive research models for over 85 years. Use of various interspecies hybrids to investigate the genetics underlying spontaneous and induced tumorigenesis has resulted in the development and maintenance of pedigreed Xiphophorus lines specifically bred for research. The recent availability of the X. maculatus reference genome assembly now provides unprecedented opportunities for novel and exciting comparative research studies among Xiphophorus species.
We present sequencing, assembly and annotation of two new genomes representing Xiphophorus couchianus and Xiphophorus hellerii. The final X. couchianus and X. hellerii assemblies have total sizes of 708 Mb and 734 Mb and correspond to 98 % and 102 % of the X. maculatus Jp 163 A genome size, respectively. The rates of single nucleotide change range from 1 per 52 bp to 1 per 69 bp among the three genomes and the impact of putatively damaging variants are presented. In addition, a survey of transposable elements allowed us to deduce an ancestral TE landscape, uncovered potential active TEs and document a recent burst of TEs during evolution of this genus.
Two new Xiphophorus genomes and their corresponding transcriptomes were efficiently assembled, the former using a novel guided assembly approach. Three assembled genome sequences within this single vertebrate order of new world live-bearing fishes will accelerate our understanding of relationship between environmental adaptation and genome evolution. In addition, these genome resources provide capability to determine allele specific gene regulation among interspecies hybrids produced by crossing any of the three species that are known to produce progeny predisposed to tumor development.
... The genus Xiphophorus (Pisces: Poeciliidae) nominally has 26 species, commonly called swordtails or platyfish, which are distributed in freshwater streams from northeastern México to eastern Honduras. In the Mexican drainages, the species are found in parts of the coastal plain and on the eastern to southeastern slopes of the Sierra Madre Oriental, but never beyond the barrier range of the Mexican plateau (Kallman and Kazianis, 2006). Several studies of the phylogeny of the genus have been conducted (e.g., Meyer et al., 1994;Marcus and McCune, 1999) and the results support the recognition of 3 principal clades. ...
... Xiphophorus malinche Rauchengerber, Kallman and Morizot, 1990 (Highland swordtail) is 1 of 9 members of the clade that inhabits the tributaries of the Río Pánuco Helminth communities of Xiphophorus malinche (Pisces: Poeciliidae), endemic freshwater fish from the Pánuco River, Hidalgo, Mexico basin. Its distribution is restricted to headwaters and highland streams of the Río Pánuco (Kallman and Kazianis, 2006;Culumber et al., 2011). Xiphophorus malinche has been the focus of the variety of behavioral ecology studies Verzijden et al., 2012), but, to date, only 4 species of Xiphophorus have been examined for helminth parasites Salgado-Maldonado, 2006), and X. malinche was not included in any of those studies. ...
... In the case of X. malinche, the species is restricted to headwaters of the Pánuco drainage, a habitat/locality that was formed during the uplifting and folding of the Sierra Madre Oriental. During this process populations became isolated in the headwaters of multiple, disconnected streams (Kallman and Kazianis, 2006). Differences in the habitat or environment, such as the aforementioned water chemistry, distance to other populations, etc., of these disconnected stream populations may account for differences in the parasite communities. ...
A total of 141 highland swordtails, Xiphophorus malinche, were collected from 2 localities in the Pánuco river drainage (Chicayotla and Malila) from Hidalgo, México. The parasite community structure of the 2 localities was examined and compared. Five taxa of helminths were recovered: 2 digeneans, adults of Paracreptotrema sp. and metacercariae of Uvulifer sp.; an adult monogenean, Urocleidoides vaginoclastrum; an adult cestode, Bothriocephalus acheilognathi, and an adult nematode, Rhabdochona xiphophori. Among them, Ur. vaginoclastrum was the most frequent and abundant species. The remaining species were rare and found at low mean abundance. Only 2 species of helminth were found at both localities. The observed species richness, individual abundance, and diversity were low at component community and infracommunity levels. Abundance of helminths and fish standard length were correlated. Uvulifer sp. was more abundant in small fish, but Ur. vaginoclastrum showed the opposite pattern, high abundance in larger fish. Differences observed in this study can be attributed to abiotic and biotic environmental factors resulting from the geographic separation of these localities during to the orogeny of the Sierra Madre Oriental that restricted fish to isolated headwaters. © 2014, Universidad Nacional Autonoma de Mexico. All rights reserved.
... La familia Poeciliidae comprende 22 géneros y 180 especies reconocidas; es una de las familias dominantes en los ambientes dulceacuícolas y salobres en tierras bajas de América Central y las Indias Occidentales, cuya distribución comprende desde el este de los Estados Unidos hasta el noreste de Argentina (Miller et al., 2005). Particularmente, el género Xiphophorus consta de 26 especies conocidas popularmente como platis y colas de espada (Kallman y Kazianis, 2006); se distribuye desde el noreste de México hasta el noreste de Honduras sobre la vertiente del Atlántico (Rosen y Bailey, 1979). ...
... Los registros de X. clemenciae corresponden a diversos ríos y arroyos que drenan hacia la presa Nezahualcóyotl (Malpaso), en los municipios de Ocozocoautla de Espinosa y Tecpatán (Fig. 1), pertenecientes a la cuenca media del río Grijalva. Habita principalmente en partes altas de los ríos y arroyos de aguas claras sombreados por la abundante vegetación ribereña, en corriente lenta a moderada y con sustrato rocoso, arenoso y hojarasca; incluso en la localidad de El Azufre, municipio de Tecpatán (Apéndice), Este constituye el primer registro de X. clemenciae fuera de la cuenca del río Coatzacoalcos, a la cual se consideraba endémica (Álvarez, 1959;Rosen y Bailey, 1979;Kallman et al., 2004;Miller et al., 2005;Kallman y Kazianis, 2006); con la finalidad de corroborar la información de su distribución, se consultaron las bases de datos de museos y colecciones disponibles en el portal electrónico Global Biodiversity Information Facility (GBIF, julio de 2013). Este hallazgo se suma a la presencia dentro de la cuenca del Grijalva de Priapella intermedia y Paraneetroplus regani, especies también consideradas con anterioridad como endémicas a la cuenca del Coatzacoalcos. ...
... La coloración en las hembras es menos conspicua, pero en ejemplares vivos y preservados pueden distinguirse porque las escamas, principalmente en la base de la aleta caudal, son muy notorias, debido a que presentan bordes oscuros, mientras que en X. hellerii las escamas son transparentes (Figs. 2, 3). Kallman y Kazianis (2006) propusieron la exclusión de X. clemenciae de las listas de especies amenazadas, debido a su abundancia y a su amplia distribución en los tributarios de la cuenca alta del río Coatzacoalcos. Con base en la ampliación del ámbito de distribución hacia la cuenca del río Grijalva y lo abundante de la especie, será necesario aplicar el método de evaluación del riesgo de extinción de las especies silvestres de México (MER) para actualizar o reconsiderar su estatus en la Norma Oficial Mexicana Nom-059- Semarnat-2010. ...
Se presenta el registro de Xiphophorus clemenciae Álvarez, 1959 para la cuenca del río Grijalva, Chiapas, México. Este es el primer registro fuera de la cuenca del río Coatzacoalcos, en los estados de Oaxaca y Veracruz, de la cual se considera endémica. Debido a lo amplio de su distribución y su alta abundancia será necesario aplicar el método de evaluación del riesgo de extinción de las especies silvestres de México (MER), para actualizar o reconsiderar su estatus en la Norma Oficial Mexicana Nom-059-Semarnat-2010.
... Xiphophorus helleri is one of the most recognized species of Xiphophorus largely due to its presence in the aquarium hobby around the world, but also within the scientific community particularly in the literature on sexual selection (Basolo 1990; Rosenthal and Evans 1998; Tatarenkov et al. 2008; Walling et al. 2008). It is currently recognized natural range -one of the broadest of any Xiphophorus -is throughout the eastern part of Mexico, bounded on the west by the Sierra Madre Oriental mountains, to the north by the Trans- Volcanic Belt, and extending as far south as Honduras and Guatemala (Kallman and Kazianis 2006). However, due to its popularity in the pet trade, feral populations of X. hellerii have been reported well outside of its range around the world from Hawaii to Australia (Poort et al. 2006; Simmons et al. 2008). ...
... If X. hellerii colonized the Barranca de Metztitlán Biosphere Reserve as a result of introduction, it is unlikely that they pose a conservation concern to other species. Species of the genus Xiphophorus often occur in sympatry with multiple other poeciliids including congeners and we know not of documented cases where competitive exclusion has replaced a native species in this group (Kallman and Kazianis 2006). For example, Poeciliopsis gracilis is non-native to the eastern side of the Sierra Madre Oriental but can be found in abundance at many lowland sites with native poeciliids such as X. birchmanni, X. variatus, Heterandria jonesii and others (ZWC pers. ...
... obs.; Obregón-Barboza et al. 1994). Hybridization within Xiphophorus is somewhat common (Rosenthal et al. 2003; Kallman and Kazianis 2006; Meyer et al. 2006; Culumber et al. 2011) and but unlikely a threat given that there are no other species of Xiphophorus present in the canyon. Based on the present genetic data we can affirm that the specimens found broadly throughout the Metztitlán Canyon Biosphere Reserve are X. ...
Biosphere reserves have been established around the world as a means to protect sensitive or threatened ecoregions and the biodiversity within them. As such, the organisms occurring within these reserves are important from a conservation perspective as they often represent endemic species or remnant populations. Here we provide genetic evidence of widespread occurrence of Xiphophorus helleri within the Metztitlán Canyon Biosphere Reserve in Hidalgo, Mexico. One nuclear and two mitochondrial genes were sequenced from four populations within the canyon and confirmed that the specimens collected were X. hellerii. When compared to published sequences of X. hellerii from multiple localities within the documented natural range of the species, the specimens from Metztitlán were found to exhibit between 0 and 1.6 % sequence divergence. Possible scenarios for colonization of the canyon and conservation implications are discussed.
... The freshwater fish genus Xiphophorus (swordtails and platyfish) belongs to the Family Poeciliidae and includes 26 species of small fish from Central America (Kallman & Kazianis 2006). This group of fish has been widely used as a model for a range of evolutionary and ecological questions including mating preferences and asymmetries, fitness differences and conservation genetics and the evolution of unisexual populations and provides remarkable opportunities for genomic studies of behavioural and ecological radiations and speciation (e.g. ...
... In a broad phytogeographic context, our molecular phylogeny provides support for the South to Central America radiation of Poeciliid fishes and the subsequent divergence of Xiphophorus species and populations (Hrbek et al. 2007). The basal split between the ancestor of the southern swordtails and the rest of the Xiphophorus radiation, and the younger origin of the platyfish clade relative to the southern swordtails (Fig. 1, Table S3, Supporting information), is in line with the known geographic distribution of these fishes (Kallman & Kazianis 2006 (Kallman & Kazianis 2006). Thus, the current distribution of this group may be limited to specific upland refugial habitats. ...
... In a broad phytogeographic context, our molecular phylogeny provides support for the South to Central America radiation of Poeciliid fishes and the subsequent divergence of Xiphophorus species and populations (Hrbek et al. 2007). The basal split between the ancestor of the southern swordtails and the rest of the Xiphophorus radiation, and the younger origin of the platyfish clade relative to the southern swordtails (Fig. 1, Table S3, Supporting information), is in line with the known geographic distribution of these fishes (Kallman & Kazianis 2006 (Kallman & Kazianis 2006). Thus, the current distribution of this group may be limited to specific upland refugial habitats. ...
Next-generation sequencing (NGS) techniques are now key tools in the detection of population genomic and gene expression differences in a large array of organisms. However, so far few studies have utilized such data for phylogenetic estimations. Here, we use NGS data obtained from genome-wide restriction site-associated DNA (RAD) (∼66000 SNPs) to estimate the phylogenetic relationships among all 26 species of swordtail and platyfish (genus Xiphophorus) from Central America. Past studies, both sequence and morphology-based, have differed in their inferences of the evolutionary relationships within this genus, particularly at the species-level and among monophyletic groupings. We show that using a large number of markers throughout the genome, we are able to infer the phylogenetic relationships with unparalleled resolution for this genus. The relationships among all three major clades and species within each of them are highly resolved and consistent under maximum likelihood, Bayesian inference and maximum parsimony. However, we also highlight the current cautions with this data type and analyses. This genus exhibits a particularly interesting evolutionary history where at least two species may have arisen through hybridization events. Here, we are able to infer the paternal lineages of these putative hybrid species. Using the RAD-marker-based tree we reconstruct the evolutionary history of the sexually selected sword trait and show that it may have been present in the common ancestor of the genus. Together our results highlight the outstanding capacity that RAD sequencing data has for resolving previously problematic phylogenetic relationships, particularly among relatively closely related species.
... Species in the genus Xiphophorus (Family Poeciliidae) are small live-bearing freshwater fish that are distributed from northern Mexico to Belize and Honduras [1,2]. Poeciliids have been widely studied in fields ranging from ecology, evolution, genetics, and genomics to systematics [3]. ...
... Although the origin and evolution of the sword [7,20,26,[53][54][55][56] and the role of hybridization in the genus Xiphophorus [2] have been addressed before, some of the phylogenetic relationships in this genus still remained uncertain. Traditionally, the genus Xiphophorus has been suggested to consist of four major lineages based on their geographical distributions and other phenotypic traits (i.e., northern platyfish, northern swordtails, southern platyfish and southern swordtails; Figure 1a) [2,11,21,22]. ...
... Although the origin and evolution of the sword [7,20,26,[53][54][55][56] and the role of hybridization in the genus Xiphophorus [2] have been addressed before, some of the phylogenetic relationships in this genus still remained uncertain. Traditionally, the genus Xiphophorus has been suggested to consist of four major lineages based on their geographical distributions and other phenotypic traits (i.e., northern platyfish, northern swordtails, southern platyfish and southern swordtails; Figure 1a) [2,11,21,22]. The monophyly and the relationships among those four lineages are not consistently supported in phylogenetic studies using molecular or combined molecular and morphological traits. ...
Background
Males in some species of the genus Xiphophorus, small freshwater fishes from Meso-America, have an extended caudal fin, or sword – hence their common name “swordtails”. Longer swords are preferred by females from both sworded and – surprisingly also, non-sworded (platyfish) species that belong to the same genus. Swordtails have been studied widely as models in research on sexual selection. Specifically, the pre-existing bias hypothesis was interpreted to best explain the observed bias of females in presumed ancestral lineages of swordless species that show a preference for assumed derived males with swords over their conspecific swordless males. However, many of the phylogenetic relationships within this genus still remained unresolved. Here we construct a comprehensive molecular phylogeny of all 26 known Xiphophorus species, including the four recently described species (X. kallmani, X. mayae, X. mixei and X. monticolus). We use two mitochondrial and six new nuclear markers in an effort to increase the understanding of the evolutionary relationships among the species in this genus. Based on the phylogeny, the evolutionary history and character state evolution of the sword was reconstructed and found to have originated in the common ancestral lineage of the genus Xiphophorus and that it was lost again secondarily.
Results
We estimated the evolutionary relationships among all known species of the genus Xiphophorus based on the largest set of DNA markers so far. The phylogeny indicates that one of the newly described swordtail species, Xiphophorus monticolus, is likely to have arisen through hybridization since it is placed with the southern platyfish in the mitochondrial phylogeny, but with the southern swordtails in the nuclear phylogeny. Such discordance between these two types of markers is a strong indication for a hybrid origin. Additionally, by using a maximum likelihood approach the possession of the sexually selected sword trait is shown to be the most likely ancestral state for the genus Xiphophorus. Further, we provide a well supported estimation of the phylogenetic relationships between the previously unresolved northern swordtail groups.
Conclusions
This comprehensive molecular phylogeny of the entire genus Xiphophorus provides evidence that a second swordtail species, X. monticolus, arose through hybridization. Previously, we demonstrated that X. clemenciae, another southern swordtail species, arose via hybridization. These findings highlight the potential key role of hybridization in the evolution of this genus and suggest the need for further investigations into how hybridization contributes to speciation more generally.
... Xiphophorus fishes belong to the internally fertilizing live-bearing family Poeciliidae. The genus ranges from northeastern Mexico to Honduras, and many species occur in sympatry (Rauchenberger et al. 1990;Kallman and Kazianis 2006). Although there is little to no intrinsic postzygotic isolation between species (Rosen 1960;Kazianis et al. 1996;Kallman and Kazianis 2006), hybridization in the wild is uncommon (reviews in Kallman and Kazianis 2006;Schartl 2008), and preferences for conspecifics as mates are a primary barrier to gene flow (Hankison and Morris 2002;Fisher et al. 2006;Kallman and Kazianis 2006;Fisher and Rosenthal 2010). ...
... The genus ranges from northeastern Mexico to Honduras, and many species occur in sympatry (Rauchenberger et al. 1990;Kallman and Kazianis 2006). Although there is little to no intrinsic postzygotic isolation between species (Rosen 1960;Kazianis et al. 1996;Kallman and Kazianis 2006), hybridization in the wild is uncommon (reviews in Kallman and Kazianis 2006;Schartl 2008), and preferences for conspecifics as mates are a primary barrier to gene flow (Hankison and Morris 2002;Fisher et al. 2006;Kallman and Kazianis 2006;Fisher and Rosenthal 2010). ...
... The genus ranges from northeastern Mexico to Honduras, and many species occur in sympatry (Rauchenberger et al. 1990;Kallman and Kazianis 2006). Although there is little to no intrinsic postzygotic isolation between species (Rosen 1960;Kazianis et al. 1996;Kallman and Kazianis 2006), hybridization in the wild is uncommon (reviews in Kallman and Kazianis 2006;Schartl 2008), and preferences for conspecifics as mates are a primary barrier to gene flow (Hankison and Morris 2002;Fisher et al. 2006;Kallman and Kazianis 2006;Fisher and Rosenthal 2010). ...
Mate choice can vary in response to internal or external conditions that alter the costs and benefits of being choosy. The relationship between mate choice and hybridization, however, is not well understood. An important influence on mate choice is the rate at which potential mates are encountered: low rates increase sampling costs, favoring reduced choosiness. We investigated the influence of conspecific encounter rate on female choice in a naturally hybridizing species of swordtail, Xiphophorus birchmanni. We exposed females to conspecific males, followed by either no delay or a long (24 h) delay before their next male encounter. In this second encounter, females were offered the choice of a heterospecific (X. malinche) male only or a choice between a conspecific and heterospecific male. When not given a choice between 2 males, females spent more time with the heterospecific following a long delay between male encounters than after no delay, suggesting a decrease in choosiness. When offered a choice between males, however, females preferentially associated with the conspecific, regardless of the time between male encounters. These results suggest that females are sensitive to conspecific encounter rate but may employ a sample-based (vs. standard-based) comparison tactic, which could make hybridization less likely. We also found that, contrary to our expectations, females with only the heterospecific male to choose from visited him more frequently following a long delay between male encounters than following no delay, possibly indicating an increase in sampling effort. Our study highlights the potential importance of context-dependent mate choice in animal hybridization. Copyright 2011, Oxford University Press.
... Fish in the genus Xiphophorus (family Poeciliidae, order Cyprinodontiformes), with known 26 species, are small freshwater fish that are native to Mexico and adjacent Central America (Rosen 1960;Kallman and Kazianis 2006). Xiphophorus fish, with common names as platyfish (Northern platy and Southern platy) and swordtails (Northern swordtail and Southern swordtail), are live-bearing fish, which have internal fertilization and ovoviviparity (Figure 1.5 ...
... Belize and Guatemala (Rosen 1960;Kallman and Kazianis 2006 (Rosen 1960;Kang et al. 2013). Likewise, sizes of males influence their reproductive tactics and the mating success in the wild. ...
Puberty is an important period of life with physiological changes to enable animals to reproduce. Xiphophorus fish exhibit polymorphism in body size, puberty timing, and reproductive tactics. These phenotypical polymorphisms are controlled by the Puberty (P) locus. In X. nigrensis and X. multilineatus, the P locus encodes the melanocortin 4 receptor (Mc4r) with high genetic polymorphisms. Mc4r is a member of the melanocortin receptors, belonging to class A G-protein coupled receptors. The Mc4r signaling system consists of Mc4r, the agonist Pomc (precursor of various MSH and of ACTH), the antagonist Agrp and accessory protein Mrap2. In humans, MC4R has a role in energy homeostasis. MC4R and MRAP2 mutations are linked to human obesity but not to puberty. Mc4rs in X. nigrensis and X. multilineatus are present in three allele classes, A, B1 and B2, of which the X-linked A alleles express functional receptors and the male-specific Y-linked B alleles encode defective receptors. Male body sizes are correlated with B allele type and B allele copy numbers. Late-maturing large males carry B alleles in high copy number while early-maturing small males carry B alleles in low copy number or only A alleles. Cell culture co-expression experiments indicated that B alleles may act as dominant negative receptor mutants on A alleles. In this study, the main aim was to biochemically characterize the mechanism of puberty regulation by Mc4r in X. nigrensis and X. multilineatus, whether it is by Mc4r dimerization and/or Mrap2 interaction with Mc4r or other mechanisms. Furthermore, Mc4r in X. hellerii (another swordtail species) and medaka (a model organism phylogenetically close to Xiphophorus) were investigated to understand if the investigated mechanisms are conserved in other species. In medaka, the Mc4r signaling system genes (mc4r, mrap2, pomc, agrp1) are expressed before hatching, with agrp1 being highly upregulated during hatching and first feeding. These genes are mainly expressed in adult brain, and the transcripts of mrap2 co-localize with mc4r indicating a function in modulating Mc4r signaling. Functional comparison between wild-type and mc4r knockout medaka showed that Mc4r knockout does not affect puberty timing but significantly delays hatching due to the retarded embryonic development of knockout medaka. Hence, the Mc4r system in medaka is involved in regulation of growth rather than puberty. In Xiphophorus, expression co-localization of mc4r and mrap2 in X. nigrensis and X. hellerii fish adult brains was characterized by in situ hybridization. In both species, large males exhibit strikingly high expression of mc4r while mrap2 shows similar expression level in the large and small male and female. Differently, X. hellerii has only A-type alleles indicating that the puberty regulation mechanisms evolved independently in Xiphophorus genus. Functional analysis of Mrap2 and Mc4r A/B1/B2 alleles of X. multilineatus showed that increased Mrap2 amounts induce higher cAMP response but EC50 values do not change much upon Mrap2 co-expression with Mc4r (expressing only A allele or A and B1 alleles). A and B1 alleles were expressed higher in large male brains, while B2 alleles were only barely expressed. Mc4r A-B1 cells have lower cAMP production than Mc4r A cells. Together, this indicates a role of Mc4r alleles, but not Mrap2, in puberty onset regulation signaling. Interaction studies by FRET approach evidenced that Mc4r A and B alleles can form heterodimers and homodimers in vitro, but only for a certain fraction of the expressed receptors. Single-molecule colocalization study using super-resolution microscope dSTORM confirmed that only few Mc4r A and B1 receptors co-localized on the membrane. Altogether, the species-specific puberty onset regulation in X. nigrensis and X. multilineatus is linked to the presence of Mc4r B alleles and to some extent to its interaction with A allele gene products. This is reasoned to result in certain levels of cAMP signaling which reaches the dynamic or static threshold to permit late puberty in large males. In summary, puberty onset regulation by dominant negative effect of Mc4r mutant alleles is a special mechanism that is found so far only in X. nigrensis and X. multilineatus. Other Xiphophorus species obviously evolved the same function of the pathway by diverse mechanisms. Mc4r in other fish (medaka) has a role in regulation of growth, reminiscent of its role in energy homeostasis in humans. The results of this study will contribute to better understand the biochemical and physiological functions of the Mc4r system in vertebrates including human.
... Currently the subfamily Poeciliinae includes 9 tribes, 29 genera and 273 species (Lucinda 2003, Reztnik et al. 2017. This subfamily contains the genus Xiphophorus Heckel, 1848, which has 28 species that occur from northeastern Mexico to northeastern Honduras in streams flowing into the Atlantic (Rosen 1960, Rosen & Bailey 1963, Kallman & Kazianis 2006, Stockwell & Henkanaththegedara 2011, Gómez-Gonzales et al. 2014. ...
... Xiphophorus helleri (Heckel, 1848) and Xiphophorus montezu mae (Jordan & Snyder, 1899), among others, and those that have not such as Xiphophorus maculatus and Xiphophorus variatus (Meek, 1904) (Kallman et al. 2004, Kallman & Kazianis 2006, Gómez-Gonzales et al. 2014. ...
... Currently the subfamily Poeciliinae includes 9 tribes, 29 genera and 273 species (Lucinda 2003, Reztnik et al. 2017. This subfamily contains the genus Xiphophorus Heckel, 1848, which has 28 species that occur from northeastern Mexico to northeastern Honduras in streams flowing into the Atlantic (Rosen 1960, Rosen & Bailey 1963, Kallman & Kazianis 2006, Stockwell & Henkanaththegedara 2011, Gómez-Gonzales et al. 2014. ...
... Xiphophorus helleri (Heckel, 1848) and Xiphophorus montezu mae (Jordan & Snyder, 1899), among others, and those that have not such as Xiphophorus maculatus and Xiphophorus variatus (Meek, 1904) (Kallman et al. 2004, Kallman & Kazianis 2006, Gómez-Gonzales et al. 2014. ...
This study reports the expansion of the known distribution of the invasive fish Xiphophorus maculatus (Günther, 1866) into the upper Magdalena river basin. The new record comes from a wetland in the Dry Tropical Forest biome in Tolima and represents the first collection of this species from the upper Magdalena river basin. An updated distribution for Colombia is provided.
... In Chapter 3, I addressed the phylogenetic relationships using a novel phylogenomic method. The genus Xiphophorus (Family Poeciliidae) includes 26 freshwater species, which are mainly distributed throughout Central America [100]. This group of fishes has widely been used as a model of evolutionary, ecological studies and oncogene research [101][102][103][104][105][106][107]. ...
... ). Different to previous work, the high support for the monophyly of the clemenciae clade recovered here strongly supports this inference. In addition, this putative paternal species overlaps in its current geographic distribution with both putative hybrid species(Kallman & Kazianis 2006), and gonopodium characters between all three species are similar(Jones et al. unpublished). Similar to the two hybrid species, X. mixei has a restricted distribution suggesting the hypothesis of a single origin for both species is likely, although this assumption is contingent on current distributions reflecting past distributions of the X. mixei lineage. ...
... Xiphophorus are a diverse group of live-bearing teleost fish consisting of 26 species with a variety of phenotypes and complex behaviors, yet little work has been done to characterize their cortisol physiology (Kallman and Kazianis, 2006). They are notable for producing fertile interspecies hybrids that are susceptible to UVB-and MNU-induced tumorigensis (Setlow et al., 1989(Setlow et al., , 1993Nairn et al., 1996;Kazianis et al., 2001aKazianis et al., , 2001b. ...
... This reasoning may have relevance to the data presented here since X. maculatus is derived from southern Mexico and may be found inhabiting stagnant and turbid waters, but also clear water streams as well. The type location for X. couchianus is described as "a spring pool" in northern Mexico around Monterrey (Kallman and Kazianis, 2006). Herein, we show X. couchianus receives more CPD damage per UVB dose and shows a lesser UVB induced primary stress response (cortisol release rate) than does X. ...
... (2004) readily found X. clemenciae in a particular area of the IT -the uplands of the Rio Coatzacoalcos systembut this species has never been found in the adjacent coastal plain. By contrast, the putative parental species, X. hellerii and X. maculatus, have the largest distribution of all Xiphophorus species extending from Veracruz, Mexico to Honduras and Guatemala, respectively, and both putative parental species overlap in their distribution with X. clemenciae (Kallman et al. 2004;Kallman & Kazianis 2006). We did not examine other potential maternal lineages, such as X. milleri , as this platyfish has a very restricted distribution that does not overlap with that of X. clemenciae (Kallman & Kazianis 2006) or X. hellerii (M. ...
... By contrast, the putative parental species, X. hellerii and X. maculatus, have the largest distribution of all Xiphophorus species extending from Veracruz, Mexico to Honduras and Guatemala, respectively, and both putative parental species overlap in their distribution with X. clemenciae (Kallman et al. 2004;Kallman & Kazianis 2006). We did not examine other potential maternal lineages, such as X. milleri , as this platyfish has a very restricted distribution that does not overlap with that of X. clemenciae (Kallman & Kazianis 2006) or X. hellerii (M. Schartl personal communication). ...
Hybrid speciation may contribute significantly to generating biodiversity, but only a few well-documented examples for it exist so far that do not involve polyploidization as a mechanism. The swordtail fish, Xiphophorus clemenciae, shows common hallmarks of a hybrid origin and still overlaps in its current geographic distribution with its putative ancestral species (Xiphophorus hellerii and Xiphophorus maculatus). Xiphophorus clemenciae provides an ideal system for investigating the possible continued genetic interactions between a hybrid and its parental species. Here, we use microsatellite and mitochondrial markers to investigate the population structure of these species of swordtails and search for signs of recent hybridization. Individuals were sampled from 21 localities across the known range of X. clemenciae- the Isthmus of Tehuantepec (IT) Mexico, and several environmental parameters that might represent barriers to dispersal were recorded. The hybridization event that gave rise to X. clemenciae appears to be rather ancient, and a single origin is likely. We find negligible evidence for ongoing hybridization and introgression between the putative ancestral species, because they now occupy distinct ecological niches, and a common haplotype is shared by most populations of X. clemenciae. The population structure within these species shows an isolation-by-distance (IBD) pattern and genetic differentiation between most populations is significant and high. We infer that tectonic evolution in the Isthmus has greatly restricted gene flow between the southern and central IT populations of X. clemenciae and X. helleriii and provide preliminary information to aid in conservation management of this geographically restricted hybrid species, X. clemenciae.
... Here we demonstrate our recent ASGE study using Xiphophorus interspecies hybrid fishes. The genus Xiphophorus has at least 27 species of live-bearing fishes found from northern Mexico south into Belize and Guatemala (Kallman and Kazianis, 2006). The Xiphophorus genus couples extreme genetic variability among Xiphophorus species with the capability of producing fertile interspecies hybrids that have allowed chromosomal inheritance of complex traits to be followed into individual F 1 and backcross hybrid progeny (; Walter and Kazianis, 2001; Meierjohann and Schartl, 2006). ...
... Table 3 shows five currently popular programs available for short read mapping that will be evaluated herein. To evaluate these programs, we used an X. maculatus reference transcriptome [for description of the species see section 2.7 and ( Walter and Kazianis, 2001;Kallman and Kazianis, 2006;Meierjohann and Schartl, 2006 Table 3. Popular short-read alignment software. ...
... occurs. Swordtail fish of the genus Xiphophorus allow us to test this hypothesis in a natural setting. Xiphophorus birchmanni and X. malinche form natural hybrids in the Sierra Madre Oriental of eastern Mexico (Rosenthal et al. 2003). X. birchmanni and X. malinche are members of the monophyletic, northern or Río Pánuco clade of swordtails (Fig. 1). Kallman & Kazianis (2006) suggested that the northern swordtails diversified as aFig. 1 Strict consensus tree based on analyses using three maximum-likelihood (ML) models implemented in PAUP. Bootstrap support values from ML analyses (GRL-Garli, PAUP and RxM- RAxML) and Bayesian posterior probabilities (BYS-MRBAYES) from separate analyses are shown for the nodes ...
... H Y B R I D Z O N E S A L O N G A N E L E V A T I O N A L G R A D I E N T 343 result of the formation of the Sierra Madre Oriental of eastern Mexico. Uplifting and subsequent folding of the landscape produced isolated species endemic to small geographic areas, like X. malinche, X. continens, X. montezumae , X. multilineatus and X. nigrensis (Rauchenberger et al. 1990; Kallman & Kazianis 2006). Phylogenetic hypotheses have differed somewhat in the placement of X. birchmanni and X. malinche. ...
Natural hybrid zones provide opportunities to study a range of evolutionary phenomena from speciation to the genetic basis of fitness-related traits. We show that widespread hybridization has occurred between two neo-tropical stream fishes with partial reproductive isolation. Phylogenetic analyses of mitochondrial sequence data showed that the swordtail fish Xiphophorus birchmanni is monophyletic and that X. malinche is part of an independent monophyletic clade with other species. Using informative single nucleotide polymorphisms in one mitochondrial and three nuclear intron loci, we genotyped 776 specimens collected from twenty-three sites along seven separate stream reaches. Hybrid zones occurred in replicated fashion in all stream reaches along a gradient from high to low elevation. Genotyping revealed substantial variation in parental and hybrid frequencies among localities. Tests of F(IS) and linkage disequilibrium (LD) revealed generally low F(IS) and LD except in five populations where both parental species and hybrids were found suggesting incomplete reproductive isolation. In these locations, heterozygote deficiency and LD were high, which suggests either selection against early generation hybrids or assortative mating. These data lay the foundation to study the adaptive basis of the replicated hybrid zone structure and for future integration of behaviour and genetics to determine the processes that lead to the population genetic patterns observed in these hybrid zones.
... Crosses varied in duration of pairing from 84 days to 1,628 days. Names from Hubbs & Hubbs (1946a) are given in parentheses next to their respective currently accepted species names (Huber, 2022, Killi-Data online;Kallman & Kazianis, 2006). Species or genera crosses that did produce offspring in at least one study are bolded. ...
The Amazon Molly ( Poecilia formosa ) reproduces by gynogenesis, a relatively rare form of asexual reproduction where sperm is required to trigger embryogenesis, but male genes are not incorporated into the genome of the embryo. Studying gynogenesis could isolate paternal non-genetic effects on reproduction. This study explored which of eleven related species can produce sperm to trigger gynogenesis through natural mating in P. formosa , and whether sympatry affects reproductive success in P. formosa . Reproductive outcomes measured were relative reproductive output (number of offspring in the first brood divided by female standard length), relative embryo output (number of embryos in the first brood divided by female standard length) and combined relative reproductive output (sum of relative reproductive output and relative embryo output). For large (>4 cm) P. formosa , combined relative reproductive output was higher with sympatric Atlantic Molly ( Poecilia mexicana ) males than with allopatric P. mexicana males. P. formosa produced live offspring or late-stage embryos with all species tested in the genera Poecilia and Limia but did not produce offspring or embryos with males from the genera Gambusia , Girardinus , Heterandria , Poeciliopsis , or Xiphophorus . This information, as well as the limitations characterized in this study, will set a foundation for use of P. formosa as a model for paternal effects and the species specificity of sperm on fertilization, embryogenesis, and reproductive success.
... In our analyses of 642 Northern swordtail individuals from sites along the river, we have identified only seventeen individuals with three-way hybrid ancestry at Tlalica, one individual at Plaza, and one individual at Calnali Low. Moreover, X. birchmanni and X. variatus are sympatric over much of X. birchmanni's range, but there has been no evidence of hybridization outside of the three-way hybrids reported here (Kallman & Kazianis, 2006). The majority of three-way hybrids we identified (~89%) were clearly first-generation crosses between X. variatus and X. malinche x X. birchmanni, pointing to a recent onset of hybridization (or reduced viability or fertility of these individuals). ...
Over the past two decades researchers have documented the extent of natural hybridization between closely related species using genomic tools. Many species across the tree of life show evidence of past hybridization with their evolutionary relatives. In some cases, this hybridization is complex - involving gene flow between more than two species. While hybridization is common over evolutionary timescales, some researchers have proposed that it may be even more common in contemporary populations where anthropogenic disturbance has modified a myriad of aspects of the environments in which organisms live and reproduce. Here, we develop a flexible tool for local ancestry inference in hybrids derived from three source populations and describe a complex, recent hybridization event between distantly related swordtail fish lineages (Xiphophorus) and its potential links to anthropogenic disturbance.
... Several studies indicate that hybridisation is common in fish (e.g. Aboim et al., 2010;Allendorf and Waples, 1996;Bagley et al., 2015;Broughton et al., 2011;Dowling et al., 2016;Harrison and Larson, 2014;Irisarri et al., 2018;Kallman and Kazianis, 2006;Meier et al., 2017;Montanari et al., 2012;Scribner et al., 2000;Svardal et al., 2020;Willis, 2017;Willis et al., 2012) and in the case of freshwater ichthyofauna, this natural phenomenon may also occur by anthropogenic disturbances in wild environments (Allendorf et al., 2010;Brennan et al., 2014;do Prado et al., 2011do Prado et al., , 2012do Prado et al., , 2017Hashimoto et al., 2014;Hasselman et al., 2014). In Pimelodidae, interspecific hybridisation has been reported for some of the most commercially important species used in Brazilian aquaculture: Pseudoplatystoma corruscans, Pseudoplatystoma reticulatum (Baggio et al., 2016) and several species of the genus Hypophthalmus (Dos Santos et al., 2019). ...
Low morphological differentiation among taxa hampers their appropriate identification, basic biological studies and promotion of any conservation effort. Aiming to provide the first insight into the evolution and speciation among north-western South American members of Pimelodus, this study tested the hypothesis that Pimelodus yuma, Pimelodus grosskopfii and Pimelodus crypticus represent three independently evolving species and explored signals of interspecific hybridisation. The outcomes based on mitochondrial (cox1) and nuclear (RADseq, microsatellites and rag2) markers combined with coalescence-based and allele-frequency methods confirmed that each studied species represents an independently evolving unit. Contrary to expectations, P. yuma was found as a sister clade of P. blochii, while P. crypticus (formerly confused with P. blochii) was phylogenetically closer to P. grosskopfii. We also found strong evidence of ancient introgression (0.66–3.32 mya) between the non-sibling species P. yuma and P. grosskopfii, breaking the absence of interbreeding and the independent evolutionary trajectory among north-western South American Pimelodus during their diversification history, a pre-requisite to define species limits. However, there were non-significant values of current gene flow between them, supporting the hypothesis of full isolation.
... Here, we describe a previously unexplored hybridization event between X. birchmanni and its more distant relative, X. cortezi (Kallman and Kazianis 2006;X. birchmanni-X. ...
Natural hybridization events provide unique windows into the barriers that keep species apart as well as the consequences of their breakdown. Here we characterize hybrid populations formed between the northern swordtail fish Xiphophorus cortezi and X. birchmanni from collection sites on two rivers. We use simulations and new genetic reference panels to develop sensitive and accurate local ancestry calling in this novel system. Strikingly, we find that hybrid populations on both rivers consist of two genetically distinct subpopulations: a cluster of pure X. birchmanni individuals and one of phenotypically intermediate hybrids that derive ∼85-90% of their genome from X. cortezi. Simulations suggest that initial hybridization occurred ∼150 generations ago at both sites, with little evidence for contemporary gene flow between subpopulations. This population structure is consistent with strong assortative mating between individuals of similar ancestry. The patterns of population structure uncovered here mirror those seen in hybridization between X. birchmanni and its sister species, X. malinche, indicating an important role for assortative mating in the evolution of hybrid populations. Future comparisons will provide a window into the shared mechanisms driving the outcomes of hybridization not only among independent hybridization events between the same species but also across distinct species pairs.
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... Mc4r signaling has been described to be of utmost importance for the puberty regulation in the laboratory model fish species Xiphophorus (Lampert et al., 2010;Volff et al., 2013). The genus Xiphophorus comprises 26 species of life-bearing freshwater fish, whose geographical distribution spreads from the Atlantic drainages of Mexico to Belize and Guatemala (Kallman and Kazianis, 2006;Rosen, 1960). Males of some species, commonly known as swordtails, develop sword-like extensions of the lower caudal fin rays, for example, X. nigrensis and X. multilineatus (both belonging to the Northern swordtail clade), and X. hellerii (belonging to the Southern swordtail clade). ...
Fish of the genus Xiphophorus provide a prominent example of genetic control of male body size and reproductive tactics. In X. nigrensis and X. multilineatus, puberty onset and body length are determined by melanocortin 4 receptor (Mc4r) allelic and copy number variations which were proposed to fine-tune the signaling output of the system. Accessory protein Mrap2 is required for growth across species by affecting Mc4r signaling. The molecular mechanism how Mc4r signaling controls puberty regulation in Xiphophorus and whether the interaction with Mrap2 is also involved was so far unclear. Hence, we examined Mc4r and Mrap2 in X. nigrensis and X. multilineatus, in comparison to a more distantly related species, X. hellerii. mc4r and mrap2 transcripts co-localized in the hypothalamus and preoptic regions in large males, small males and females of X. nigrensis, with similar signal strength for mrap2 but higher expression of mc4r in large males. This overexpression is constituted by wild-type and one subtype of mutant alleles. In vitro studies revealed that Mrap2 co-expressed with Mc4r increased cAMP production but did not change EC50. Cells co-expressing the wild-type and one mutant allele showed lower cAMP signaling than Mc4r wild-type cells. This indicates a role of Mc4r alleles, but not Mrap2, in puberty signaling. Different from X. nigrensis and X. multilineatus, X. hellerii has only wild-type alleles, but also shows a puberty onset and body length polymorphism, despite the absence of mutant alleles. Like in the two other species, mc4r and mrap2 transcripts colocalized and mc4r is expressed at substantially higher levels in large males. This demonstrates that puberty and growth regulation mechanism may not be identical even within same genus.
... Xiphophorus multilineatus is one of nine species belonging to the Northern Swordtail clade within Xiphophorus (Rauchenberger et al., 1990;Kallman & Kazianis, 2006;Kang et al., 2013). Two other species in the Northern Swordtail clade, Xiphophorus continens and Xiphophorus pygmaeus, have evolutionarily lost the large male size classes and have lost courtship behaviour, using only force copulatory sneak chases to mate (Ryan & Causey, 1989;Morris et al., 2005). ...
Alternative reproductive tactics (ARTs) are characterized by dimorphism for reproductive phenotypes (i.e. tactical dimorphism). Tactical dimorphism often evolves as a response to differing phenotypic optima for a shared reproductive trait between members of the same sex in a species. We characterized dimorphism for body shape between ARTs in the swordtail fish Xiphophorus multilineatus and investigated the relationship between body shape and mating behaviours. Xiphophorus multilineatus possesses two ARTs, a courter male that uses only courtship to attract females, and a sneaker male that is contextually plastic and uses both courtship and force copulatory sneak chases. We found that courter and sneaker males are dimorphic for body shape; however, we did not detect a significant difference in the allometric relationship between body shape and size. Additionally, we found that more fusiform sneaker males were more likely to use sneak-chase behaviour in the absence of a competitor. These results suggest that the ARTs in X. multilineatus have different phenotypic optima for body shape and that a fusiform body shape might be more optimal for sneak-chase behaviour. Understanding tactical dimorphism in this species will allow us to explore further how the ARTs evolved and the potential for intralocus tactical conflict to constrain the evolution of each ART from reaching their respective optima. ADDITIONAL KEYWORDS: alternative reproductive tactics-body shape-geometric morphometrics-intralocus tactical conflict-mating behaviour-phenotypic optima-tactical dimorphism-Xiphophorus multilineatus.
... Xiphophorus hellerii present an anteriorly rounded and posteriorly compressed body; depressed head; large cycloid scales distributed from the head to the body; large dorsal-fin; and a small, terminal, and prognathous mouth (Esmaeili, Gholamifard, Teimori, Baghbani, & Coad, 2010). This species is naturally distributed from northeastern Mexico (Coahuila) to Honduras, Central America (Kallman & Kazianis, 2006), but has been introduced into at least 33 countries and territories (Esmaeili et al., 2010), including Brazil. In this study, this exotic non-native species is reported for the first time in the lower Iguazu River Basin in the South region of Brazil. ...
... The 26 known species of Xiphophorus are tropical, ranging from northern Mexico, and south along the Sierra Madre uplift, into Belize and Honduras [25]. These fish are known for a variety of expressed pigment patterns and these patterns are thought to play a role in complex courting behavior necessary for live-bearing animals to select mates [26]. ...
Background:
Evolution occurred exclusively under the full spectrum of sunlight. Conscription of narrow regions of the solar spectrum by specific photoreceptors suggests a common strategy for regulation of genetic pathways. Fluorescent light (FL) does not possess the complexity of the solar spectrum and has only been in service for about 60 years. If vertebrates evolved specific genetic responses regulated by light wavelengths representing the entire solar spectrum, there may be genetic consequences to reducing the spectral complexity of light.
Results:
We utilized RNA-Seq to assess changes in the transcriptional profiles of Xiphophorus maculatus skin after exposure to FL ("cool white"), or narrow wavelength regions of light between 350 and 600 nm (i.e., 50 nm or 10 nm regions, herein termed "wavebands"). Exposure to each 50 nm waveband identified sets of genes representing discrete pathways that showed waveband specific transcriptional modulation. For example, 350-400 or 450-500 nm waveband exposures resulted in opposite regulation of gene sets marking necrosis and apoptosis (i.e., 350-400 nm; necrosis suppression, apoptosis activation, while 450-500 nm; apoptosis suppression, necrosis activation). Further investigation of specific transcriptional modulation employing successive 10 nm waveband exposures between 500 and 550 nm showed; (a) greater numbers of genes may be transcriptionally modulated after 10 nm exposures, than observed for 50 nm or FL exposures, (b) the 10 nm wavebands induced gene sets showing greater functional specificity than 50 nm or FL exposures, and (c) the genetic effects of FL are primarily due to 30 nm between 500 and 530 nm. Interestingly, many genetic pathways exhibited completely opposite transcriptional effects after different waveband exposures. For example, the epidermal growth factor (EGF) pathway exhibits transcriptional suppression after FL exposure, becomes highly active after 450-500 nm waveband exposure, and again, exhibits strong transcriptional suppression after exposure to the 520-530 nm waveband.
Conclusions:
Collectively, these results suggest one may manipulate transcription of specific genetic pathways in skin by exposure of the intact animal to specific wavebands of light. In addition, we identify genes transcriptionally modulated in a predictable manner by specific waveband exposures. Such genes, and their regulatory elements, may represent valuable tools for genetic engineering and gene therapy protocols.
... The TMBV began its formation 20 million years ago, and it has been in change until recent times (Ferrari, Orozco-Esquivel, Manea, & Manea, 2012). This province has been associated with the formation and/or diversification of various groups of plants (G andara & Sosa, 2014), reptiles (Bryson, Garc ıa-V azquez, & Riddle, 2012a, 2012b, fishes (Kallman & Kazianis, 2006;Mateos, 2005; Ornelas-Garc ıa, Dom ınguez-Dom ınguez, & Doadrio, 2008), and even other orthopterans (Pedraza-Lara, Barrientos-Lozano, Rocha-S anchez, & Zald ıvar-River on, 2015). ...
Taeniopoda is a genus of grasshoppers currently represented by 12 species distributed from southern USA to Panama, with most of them occurring along the transitional Nearctic–Neotropical region in central and southern Mexico. Despite being a small group of conspicuous, colourful species, the systematics of Taeniopoda has been largely neglected, including its phylogenetic affinity with the morphologically similar, monotypic genus Romalea. Here, we assessed the species limits in 11 of the species of Teniopoda based on two mitochondrial (mt) markers (COI, cyt b). Phylogenetic relationships were reconstructed adding two nuclear gene markers (28S, H3). A relaxed molecular clock analysis was performed based on the mt markers. We detected nuclear mt paralogues (numts) and the probable introgression of T. tamaulipensis mtDNA in specimens of T. eques from central Mexico. Between six and 14 species of Taeniopoda were delimited by the sequence-based approaches performed (COI divergence with thresholds of 1 and 2%; General Mixed Yule-Coalescent (GMYC) model). The GMYC and 1% threshold analyses with COI were more congruent with the currently recognized morphology-based taxonomy with 10 and 11 putative species, respectively. Four of these species were regarded as ‘stable’, since they were supported by at least one of the molecular analyses and by diagnostic morphological features. The species-based phylogeny recovered Taeniopoda as paraphyletic with respect to the monotypic genus Romalea. Three morphologically and geographically congruent major clades were recovered, two with species having a considerably elevated pronotal crest and one with its members having it less elevated. The origin and subsequent diversification of Taeniopoda were estimated to occur from the mid and late Miocene to Pliocene, respectively. The current species diversity in Taeniopoda was estimated to occur during the Pleistocene, which was probably influenced by the climatic oscillations that occurred during this period and the uplift of mountain ranges in Central America.
... The distribution of X. malinche is restricted to the more northern Hidalgo anticline, separated from southern populations of fish by the barrier range of the Mexican plateau (Kallman and Kazianis 2006), so it is not clear how the population of P. rosenthali sp. n. is linked to those species of Central America. ...
... The distribution of X. malinche is restricted to the more northern Hidalgo anticline, separated from southern populations of fish by the barrier range of the Mexican plateau (Kallman and Kazianis 2006), so it is not clear how the population of P. rosenthali sp. n. is linked to those species of Central America. ...
Paracreptotrema
rosenthali
sp. n. was discovered in the intestine of Xiphophorus
malinche and Pseudoxiphophorus
jonesii, collected from the headwaters of Río Malila, tributary of Río Conzintla, in the Río Pánuco basin, Hidalgo, México, during 2008–2009. The new species differs from the five known species of Paracreptotrema Choudhury, Pérez-Ponce de León, Brooks & Daverdin, 2006 by having vitelline follicles that extend from a level anterior to the pharynx to mid-testes, the seminal vesicle which is more extensively folded, and a wider cirrus sac. The new species resembles Paracreptotrema
heterandriae in the length of its ceca, which surpasses the posterior margin of the ovary but do not reach the testes. A key to the species of Paracreptotrema is provided.
... Species of this genus that carry swords are called swordtails, while nonsworded species are colloquially referred to as platyfish. Within swordtails, the swords vary: some species have very long extended colorful swords that can be longer than the body of the males, whereas others have only short and colorless ventral protrusions in the caudal fin (Rosen 1960;Kallman and Kazianis 2006). Some, swordtails that also tend to have more slender bodies than platies, for example, X. pygmaeus, and X. continens, do not have much of a sword at all (Meyer et al. 1994;Meyer 1997). ...
Swords are exaggerated male ornaments of swordtail fishes that have been of great interest to evolutionary biologists ever since Darwin described them in the Descent of Man (1871). They are a novel sexually selected trait derived from modified ventral caudal fin rays and are only found in the genus Xiphophorus. Another phylogenetically more widespread and older male trait is the gonopodium, an intromittent organ found in all poeciliid fishes, that is derived from a modified anal fin. Despite many evolutionary and behavioral studies on both traits, little is known so far about the molecular mechanisms underlying their development. By investigating transcriptomic changes (utilizing a RNA-Seq approach) in response to testosterone treatment in the swordtail fish, Xiphophorus hellerii, we aimed to better understand the architecture of the gene regulatory networks underpinning the development of these two evolutionary novelties. Large numbers of genes with tissue-specific expression patterns were identified. Among the “sword genes” those involved in embryonic organ development, sexual character development and coloration were highly expressed, while in the gonopodium rather more morphogenesis-related genes were found. Interestingly, many genes and genetic pathways are shared between both developing novel traits derived from median fins: the sword and the gonopodium. Our analyses show that a larger set of gene networks was co-opted during the development and evolution of the “older” gonopodium than in the “younger,” and morphologically less complex trait, the sword. We provide a catalog of candidate genes for future efforts to dissect the development of those sexually selected exaggerated male traits in swordtails.
... Xiphophorus species are thought to have diverged ≈4-6 million years ago (Mateos and Vrijenhoek, 2002;Kallman and Kazianis, 2006) and the two species used in the studies reported, X. maculatus and X. couchianus, segregate into divergent clades in most recent phylogenetic reconstructions of this genus (Cui et al., 2013;Kang Hyoun et al., 2013). In addition, sites where these two species were collected ranged from Southern (X. maculatus, Rio Jamapa, Veracruz) and Northern (X. ...
... Crosses between two Xiphophorus species provided the first example of a BDM incompatibility (Kosswig 1928; Gordon 1931). Xiphophorus maculatus and X. hellerii are sympatric, and they occasionally hybridize in nature (Kallman and Kazianis 2006; Meyer et al. 2006; Rosenthal and GarciaDe-Leon 2011; Kang et al. 2013). Xiphophorus maculatus individuals carry an oncogene and its repressor locus, both of which are absent or nonfunctional in X. hellerii. ...
Genetic incompatibilities are commonly observed between hybridizing species. Although this type of isolating mechanism has
received considerable attention, we have few examples describing how genetic incompatibilities evolve. We investigated the
evolution of two loci involved in a classic example of a Bateson–Dobzhansky–Muller (BDM) incompatibility in Xiphophorus, a genus of freshwater fishes from northern Central America. Hybrids develop a lethal melanoma due to the interaction of
two loci, an oncogene and its repressor. We cloned and sequenced the putative repressor locus in 25 Xiphophorus species and an outgroup species, and determined the status of the oncogene in those species from the literature. Using phylogenetic
analyses, we find evidence that a repeat region in the proximal promoter of the repressor is coevolving with the oncogene.
The data support a hypothesis that departs from the standard BDM model: it appears the alleles that cause the incompatibilities
have coevolved simultaneously within lineages, rather than in allopatric or temporal isolation.
... Despite their increasing use in population genetics and gene mapping studies, next-generation sequencing techniques have had limited applications in phylogenetics thus far. Though a variety of next-generation sequencing methods can be used generate phylogenetic datasets, for recently diverged species such as Xiphophorus (~4-6 mya; Mateos et al. 2002; Kallman and Kazianis 2006), techniques such as ...
Hybridization is increasingly being recognized as a widespread process, even between ecologically and behaviorally divergent animal species. Determining phylogenetic relationships in the presence of hybridization remains a major challenge for evolutionary biologists, but advances in sequencing technology and phylogenetic techniques are beginning to address these challenges. Here we reconstruct evolutionary relationships among swordtails and platyfishes (Xiphophorus: Poeciliidae), a group of species characterized by remarkable morphological diversity and behavioral barriers to interspecific mating. Past attempts to reconstruct phylogenetic relationships within Xiphophorus have produced conflicting results. Because many of the 26 species in the genus are interfertile, these conflicts are likely due to hybridization. Using genomic data, we resolve a high-confidence species tree of Xiphophorus that accounts for both incomplete lineage sorting and hybridization. Our results allow us to reexamine a long-standing controversy about the evolution of the sexually selected sword in Xiphophorus, and demonstrate that hybridization has been strikingly widespread in the evolutionary history of this genus.
... For instance the evolutionary process of speciation and separation of species along an elevational gradient is well documented in some species pairs of the poeciliid genus Xiphophorus (e.g. X. malinche and X. birchmanni; X. clemenciae and X. helleri) from the Atlantic slopes of the Sierra Madre in Mexico (Kallman & Kazianis, 2006;Culumber et al., 2010). Thus, it is not unlikely that R. parlettei evolved from a peripherally isolated allopatric population of its putative sister species R. christinae. ...
Rivulus parlettei spec. nov. from south eastern Peru (Departamento Cusco) is described. It is a member of the R. limoncochae group (as diagnosed by Costa, 2010) and differs from all the other species of this assemblage by its unique colour pattern and different morphology. It is distinguished from its geographic neighbour R. christinae by irregular interrupted lines of dots wider than the interspaces on body sides (versus longitudinal rows of dots forming stripes narrower than the interspaces). The influence of the occupation of distinct microhabitats for the radiation of the clades and the possible process of speciation of the new species are briefly discussed.
... Despite their increasing use in population genetics and gene mapping studies, next-generation sequencing techniques have had limited applications in phylogenetics thus far. Although a variety of next-generation sequencing methods can be used to generate phylogenetic datasets, for recently diverged species such as Xiphophorus (∼4-6 mya; Mateos et al. 2002;Kallman and Kazianis 2006), techniques such as RNAseq that generate long alignments are promising (e.g., Kocot et al. 2011;Smith et al. 2011). Although RNAseq datasets have been concatenated to produce highly supported phylogenies (Hittinger et al. 2010;Nabholz et al. 2011), concatenated datasets cannot be used to investigate gene flow. ...
Hybridization is increasingly being recognized as a widespread process, even between ecologically and behaviorally divergent animal species. Determining phylogenetic relationships in the presence of hybridization remains a major challenge for evolutionary biologists, but advances in sequencing technology and phylogenetic techniques are beginning to address these challenges. Here we reconstruct evolutionary relationships among swordtails and platyfishes (Xiphophorus: Poeciliidae), a group of species characterized by remarkable morphological diversity and behavioral barriers to interspecific mating. Past attempts to reconstruct phylogenetic relationships within Xiphophorus have produced conflicting results. Since many of the 26 species in the genus are interfertile, these conflicts are likely due to hybridization. Using genomic data, we resolve a high-confidence species tree of Xiphophorus that accounts for both incomplete lineage sorting and hybridization. Our results allow us to re-examine a long-standing controversy about the evolution of the sexually selected sword in Xiphophorus, and demonstrate that hybridization has been strikingly widespread in the evolutionary history of this genus.
DOI: 10.1111/evo.12099
... Although distinguishing between introgression and hybrid trait speciation remains challenging, the first step in investigating the role of past hybridization in speciation is determining how much of the genome can be attributed to hybrid origin. The genus Xiphophorus is composed of 26 species of freshwater fish remarkable both for their striking morphological diversity driven by sexual selection and for the broad interfertility between reproductively isolated species in this genus (Kazianis et al. 1996; Kallman and Kazianis 2006). Xiphophorus is made up of three monophyletic groups: the southern swordtails, the northern swordtails, and the platyfish (Fig. 1A). ...
Once thought rare in animal taxa, hybridization has been increasingly recognized as an important and common force in animal evolution. In the past decade, a number of studies have suggested that hybridization has driven speciation in some animal groups. We investigate the signature of hybridization in the genome of a putative hybrid species, Xiphophorus clemenciae, through whole genome sequencing of this species and its hypothesized progenitors. Based on analysis of this data, we find that X. clemenciae is unlikely to have been derived from admixture between its proposed parental species. However, we find significant evidence for recent gene flow between Xiphophorus species. Although we detect genetic exchange in two pairs of species analyzed, the proportion of genomic regions that can be attributed to hybrid origin is small, suggesting that strong behavioral premating isolation prevents frequent hybridization in Xiphophorus. The direction of gene flow between species is potentially consistent with a role for sexual selection in mediating hybridization.
... G. aurata, G. speciosa, G. vittata, P. latipunctata, X. variatus and Xiphophorus xiphidium (Gordon 1932). Combined with the importance of the Trans-Mexican Volcanic Belt and the Los Tuxtlas volcanoes in areas south of the study area examined here (Miller & Smith, 1986; Obregón-Barboza et al., 1994; Contreras-Balderas et al., 1996; Mateos et al., 2002; Hulsey et al., 2004; Kallman & Kazianis, 2006; McEachran & Dewitt, 2008), this suggests a major role for late Miocene and Pliocene orogenesis in the complex biogeography of freshwater fishes in east-central México. Gambusia quadruncus differs from its closest relative, G. affinis, in several characters that could prevent significant interbreeding when sympatric. ...
Gambusia quadruncus n. sp., the llanos mosquitofish, is described from east-central México. The region inhabited by the species represents a hotspot of diversity of Gambusia, and G. quadruncus sometimes coexists with at least three congeners. The species differs from its closest relative, Gambusia affinis, in several characteristics with plausible effects on reproductive isolation, e.g. body size, body and fin morphology, male genital morphology (distal tip of gonopodium) and female anal spot morphology (colouration near the urogenital sinus). Moreover, combined analysis of mitochondrial and nuclear gene sequence data (c. 2158 total base pairs) indicates reciprocal monophyly of G. quadruncus and its sister species G. affinis, with levels of genetic divergence suggesting the two species diverged from one another over a million years ago. The origin of G. quadruncus may reflect a vicariant event associated with Pliocene orogenesis in the Tamaulipas Arch and a frontal section of the Sierra Madre Oriental (Lleran Mesas). Gambusia quadruncus inhabits a variety of freshwater habitats across several river drainages, with its range spanning at least 350 km from north to south, covering over 25 000 km(2) . A key to aid identification of the species is provided.
... Most Xiphophorus species are native to Mexico, Belize, Guatemala, and Honduras. 5 The use of these fishes as a research model began in the mid-1920's, and lead to the establishment of the Xiphophorus Genetic Stock Center (XGSC) in 1939. 6 After more than 70 years of effort, the XGSC currently maintains 58 pedigreed lines among 24 of the 27 known Xiphophorus species. ...
Abstract Cryopreservation of sperm from Xiphophorus fishes has produced live young in three species: X. hellerii, X. couchianus, and X. maculatus. In this study, the goal was to establish protocols for sperm cryopreservation and artificial insemination to produce live young in X. variatus, and to identify needs for repository development. The objectives were to: 1) collect basic biological characteristics of males; 2) cryopreserve sperm from X. variatus, 3) harvest live young from cryopreserved sperm, and 4) discuss the requirements for establishment of sperm repositories. The 35 males used in this study had a body weight of 0.298±0.096 g (mean±SD), body length of 2.5±0.2 cm, and testis weight of 6.4±3.4 mg. The sperm production per gram of testis was 2.33±1.32×10(9) cells. After freezing, the post-thaw motility decreased significantly to 37%±17% (ranging from 5% to 70%) (p=0.000) from 57%±14% (40%-80%) of fresh sperm (N=20). Artificial insemination of post-thaw sperm produced confirmed offspring from females of X. hellerii and X. variatus. This research, taken together with previous studies, provides a foundation for development of strategies for sperm repositories of Xiphophorus fishes. This includes: 1) the need for breeding strategies for regeneration of target populations, 2) identification of minimum fertilization capacity of frozen samples, 3) identification of fish numbers necessary for sampling and their genetic relationships, 4) selection of packaging containers for labeling and biosecurity, 5) assurance of quality control and standardization of procedures, 6) information systems that can manage the data associated with cryopreserved samples, including the genetic data, 7) biological data of sampled fish, 8) inventory data associated with frozen samples, and 9) data linking germplasm samples with other related materials such as body tissues or cells saved for DNA and RNA analyses.
... Although many other species of Xiphophorus live in sympatry with others and can hybridize in the lab, hybridization has ony been reported for a few species pairs, and in all cases appears rare (reviewed in [52]). In X. birchmanni, both organic pollution [32] and encounter rates with conspecific males [34] reduce female discrimination against X. malinche males, in addition to predation risk as reported here. ...
Mate choice is context dependent, but the importance of current context to interspecific mating and hybridization is largely unexplored. An important influence on mate choice is predation risk. We investigated how variation in an indirect cue of predation risk, distance to shelter, influences mate choice in the swordtail Xiphophorus birchmanni, a species which sometimes hybridizes with X. malinche in the wild. We conducted mate choice experiments to determine whether females attend to the distance to shelter and whether this cue of predation risk can counteract female preference for conspecifics. Females were sensitive to shelter distance independent of male presence. When conspecific and heterospecific X. malinche males were in equally risky habitats (i.e., equally distant from shelter), females associated primarily with conspecifics, suggesting an innate preference for conspecifics. However, when heterospecific males were in less risky habitat (i.e., closer to shelter) than conspecific males, females no longer exhibited a preference, suggesting that females calibrate their mate choices in response to predation risk. Our findings illustrate the potential for hybridization to arise, not necessarily through reproductive "mistakes", but as one of many potential outcomes of a context-dependent mate choice strategy.
... Several new families of TEs in fish have been identified for the first time on the sex chromosomes of the platyfish Xiphophorus maculatus. This poeciliid freshwater fish belongs to a genus currently comprising 26 species inhabiting Atlantic drainages of Mexico, Guatemala, and Honduras (Kallman and Kazianis 2006). Xiphophorus has been used as a model to study melanoma formation (Gordon 1927;Kosswig 1928;Meierjohann and Schartl 2006), sexual selection (Offen et al. 2009), reproduction (Cummings and Gelineau-Kattner 2009), behavior (Fernandez et al. 2008), and sex determination and sexual development Böhne et al. 2009;Lampert et al. 2010). ...
Transposable elements are widespread mobile DNA sequences able to integrate into new locations within genomes. Through transposition and recombination, they significantly contribute to genome plasticity and evolution. They can also regulate gene expression and provide regulatory and coding sequences (CDSs) for the evolution of novel gene functions. We have identified a new superfamily of DNA transposon on the Y chromosome of the platyfish Xiphophorus maculatus. This element is 11 kb in length and carries a single CDS of 24 exons. The N-terminal part of the putative protein, which is expressed in all adult tissues tested, contains several nucleic acid- and protein-binding domains and might correspond to a novel type of transposase/integrase not described so far in any transposon. In addition, a testis-specific splice isoform encodes a C-terminal Ulp1 SUMO protease domain, suggesting a function in posttranslational protein modification mediated by SUMO and/or ubiquitin small peptides. Accordingly, this element was called Zisupton, for Zinc finger SUMO protease transposon. Beside the Y-chromosomal sequence, five other very similar copies were identified in the platyfish genome. All copies are delimited by 99-bp conserved subterminal inverted repeats and flanked by copy-specific 8-nt target site duplications reflecting their integration at different positions in the genome. Zisupton elements are inserted at different genomic locations in different poeciliid species but also in different populations of X. maculatus. Such insertion polymorphisms between related species and populations indicate relatively recent transposition activity, with a high degree of nucleotide identity between species suggesting possible implication of horizontal gene transfer. Zisupton sequences were detected in other fish species, in urochordates, cephalochordates, and hemichordates as well as in more distant organisms, such as basidiomycete fungi, filamentous brown algae, and green algae. Possible examples of nuclear genes derived from Zisupton have been identified. To conclude, our analysis has uncovered a new superfamily of DNA transposons with potential roles in genome diversity and evolutionary innovation in fish and other organisms.
... The genus Xiphophorus (Poeciliidae) consists of 26 species (Kallman and Kazianis 2006) that are divided into species with swords, the swordtails, and the typically sword-less platyfishes. In adult swordtail males a set of ventral fin rays of the caudal fin are elongated and often ornamented with bright, contrasting pigmentation, forming the so-called sword (for definitions of which traits constitute a sword, see Basolo 1991; Wiens and Morris 1996; Meyer 1997). ...
Male swordtail fish of the genus Xiphophorus (Poeciliidae) possess a "sword" that is composed of several colored elongated ventral fin rays of the caudal fin. The sword is a secondary sexual trait that evolved through sexual selection by female preference. To uncover the developmental mechanisms underlying the metamorphosis from a juvenile caudal fin to the sword, we have devised a transplantation protocol to assay the fate of single transplanted fin rays and their interactions with flanking rays. These experiments provide evidence for the existence of a previously unrecognized inductive signal that originates in those rays that develop into the two longest sword rays. This "sword organizer" causes adjacent fin rays to grow and become integrated into the sword and induces the development of an additional, typically pigmented sword in grafts to the dorsal part of the caudal fin. We show that the potential to develop a sword is restricted to certain parts of the caudal fin. Our findings suggest that the evolution of swords in swordtails required the acquisition of two developmental mechanisms: the establishment of signaling competence in prospective sword rays in the embryo or early larva, and its activation through androgen signaling in adult male fish.
... maculatus Jp 163) collected from the wild. Upon establishment in the XGSC the X. maculatus Jp 163 line was spilt into Jp 163 A (Sd) and B (Sp) lines based on the pigment patterns and have been maintained as two independent pedigrees for over 100 generations (Kallman 2001, Kallman and Kazianis, 2006, Walter et al. 2006a. First generation hybrids (F 1 ) from three interspecies crosses were utilized in the studies presented. ...
Research investigating telomere lengths and telomerase expression in vertebrates has progressively become important due to the association of these two biological endpoints with cellular aging and cancer in humans. Studies that rely upon the traditional use of laboratory mice have been faced with limitations largely due to inbred mice possessing large telomeres and ubiquitous expression of telomerase. Recently, a number of small fish species have been shown to provide potentially informative models for examining the role of telomeres and telomerase within intact vertebrate animals. Xiphophorus fishes represent a new world live-bearing genus that has not previously been assessed for telomere length or telomerase expression. To add to the knowledge base of telomere and telomerase biology in vertebrates we assessed telomere length and telomerase expression among several species of Xiphophorus. The telomere lengths in several organs (gill, brain, eyes, testis, ovary and liver) in three species (Xiphophorus hellerii, Xiphophorus maculatus, Xiphophorus couchianus) and also in F1 interspecies hybrids were approximately 2–6 kb. This size was consistent within the same organs of the same species, as well as between species and F1 hybrids. Despite possessing relatively short telomere lengths compared to humans, the consistency of size among Xiphophorus species and organs may allow experimental detection of telomere shortening.
... The genus Xiphophorus has at least 27 species of live-bearing fishes found from northern Mexico south into Belize and Guatemala (Kallman and Kazianis, 2006). The Xiphophorus genus couples extreme genetic variability among Xiphophorus species with the capability of producing fertile interspecies hybrids that have allowed chromosomal inheritance of complex traits to be followed into individual F 1 and backcross hybrid progeny. ...
Variations in gene expression are essential for the evolution of novel phenotypes and for speciation. Studying allelic specific gene expression (ASGE) within interspecies hybrids provides a unique opportunity to reveal underlying mechanisms of genetic variation. Using Xiphophorus interspecies hybrid fishes and high-throughput next generation sequencing technology, we were able to assess variations between two closely related vertebrate species, Xiphophorus maculatus and Xiphophorus couchianus, and their F(1) interspecies hybrids. We constructed transcriptome-wide SNP polymorphism sets between two highly inbred X. maculatus lines (JP 163 A and B), and between X. maculatus and a second species, X. couchianus. The X. maculatus JP 163 A and B parental lines have been separated in the laboratory for ≈70 years and we were able to identify SNPs at a resolution of 1 SNP per 49 kb of transcriptome. In contrast, SNP polymorphisms between X. couchianus and X. maculatus species, which diverged ≈5-10 million years ago, were identified about every 700 bp. Using 6524 transcripts with identified SNPs between the two parental species (X. maculatus and X. couchianus), we mapped RNA-seq reads to determine ASGE within F(1) interspecies hybrids. We developed an in silico X. couchianus transcriptome by replacing 90,788 SNP bases for X. maculatus transcriptome with the consensus X. couchianus SNP bases and provide evidence that this procedure overcomes read mapping biases. Employment of the in silico reference transcriptome and tolerating 5 mismatches during read mapping allow direct assessment of ASGE in the F(1) interspecies hybrids. Overall, these results show that Xiphophorus is a tractable vertebrate experimental model to investigate how genetic variations that occur during speciation may affect gene interactions and the regulation of gene expression.
Because of diverged adaptative phenotypes, fish species of the genus Xiphophorus have contributed to a wide range of research for a century. Existing Xiphophorus genome assemblies are not at the chromosomal level and are prone to sequence gaps, thus hindering advancement of the intra- and inter-species differences for evolutionary, comparative, and translational biomedical studies. Herein, we assembled high-quality chromosome-level genome assemblies for three distantly related Xiphophorus species, namely, X. maculatus , X. couchianus , and X. hellerii . Our overall goal is to precisely assess microevolutionary processes in the clade to ascertain molecular events that led to the divergence of the Xiphophorus species and to progress understanding of genetic incompatibility to disease. In particular, we measured intra- and inter-species divergence and assessed gene expression dysregulation in reciprocal interspecies hybrids among the three species. We found expanded gene families and positively selected genes associated with live bearing, a special mode of reproduction. We also found positively selected gene families are significantly enriched in nonpolymorphic transposable elements, suggesting the dispersal of these nonpolymorphic transposable elements has accompanied the evolution of the genes, possibly by incorporating new regulatory elements in support of the Britten–Davidson hypothesis. We characterized inter-specific polymorphisms, structural variants, and polymorphic transposable element insertions and assessed their association to interspecies hybridization-induced gene expression dysregulation related to specific disease states in humans.
Over the past two decades researchers have documented the extent of natural hybridization between closely related species using genomic tools. Many species across the tree of life show evidence of past hybridization with their evolutionary relatives. In some cases, this hybridization is complex – involving gene flow between more than two species. While hybridization is common over evolutionary timescales, some researchers have proposed that it may be even more common in contemporary populations where anthropogenic disturbance has modified myriad aspects of the environments in which organisms live and reproduce. Here, we develop a flexible tool for local ancestry inference in hybrids derived from three source populations and describe a complex, recent hybridization event between distantly related swordtail fish lineages ( Xiphophorus ) and its potential links to anthropogenic disturbance.
Impact Summary
As sequencing tools have advanced, we have found that barriers between animal species are more porous than once thought. Researchers have found evidence for hybridization between species throughout many branches of the tree of life. In some cases, these hybridization events can involve more than two species. Here, we develop a flexible and user-friendly tool that can be used to identify three-way hybrids and report the discovery of hybrids with ancestry from three swordtail ( Xiphophorus ) species from an anthropogenically impacted site on the Río Calnali in Hidalgo, Mexico. Researchers have studied hybrids between two Xiphophorus species along this river for decades, but this is the first documented case of hybridization involving three species. We explore hypotheses for what drove this hybridization event, including anthropogenic pollutants and reduced water quality.
Natural hybridization events provide unique windows into the barriers that keep species apart as well as the consequences of their breakdown. Here we characterize hybrid populations formed between the northern swordtail fish Xiphophorus cortezi and X. birchmanni from collection sites on two rivers. We develop sensitive and accurate local ancestry calling for this system based on low coverage whole genome sequencing. Strikingly, we find that hybrid populations on both rivers consist of two genetically distinct subpopulations: a cluster of nearly pure X. birchmanni individuals and one of phenotypically intermediate hybrids that derive ~85-90% of their genome from X. cortezi . Simulations and empirical data suggest that at both sites initial hybridization occurred ~150 generations ago, with little evidence for contemporary gene flow between subpopulations, likely due to strong assortative mating. The patterns of population structure uncovered here mirror those seen in hybridization between X. birchmanni and its sister species, X. malinche . Future comparisons will provide a window into the repeatability of the outcomes of hybridization not only across independent hybridization events between the same species but also across distinct species pairs.
The role of ecology in phenotypic and species diversification is widely documented. Nonetheless, numerous non-adaptive processes can shape realized niches and phenotypic variation in natural populations, complicating inferences about adaptive evolution at macroevolutionary scales. We tested for evolved differences in thermal tolerances and their association with the realized thermal niche (including metrics describing diurnal and seasonal patterns of temperature extremes and variability) across a genus of tropical freshwater fishes reared in a standardized environment. There was limited evolution along the thermal niche axis associated with variation in maximum temperature and of upper thermal limits. In contrast, there was considerable diversification along the first major axis of the thermal niche associated with minimum temperatures and in lower thermal limits. Across our adaptive landscape analyses, 70% of species exhibited evidence of divergence in thermal niches. Most importantly, the first two major axes of thermal niche variation were significantly correlated with variation in lower thermal limits. Our results indicate adaptation to divergent thermal niches and adaptive evolution of related functional traits, and highlight the importance of divergence in lower thermal limits for the evolution of tropical biodiversity.
Background
Nutritional programming takes place in early development. Variation in the quality and/or quantity of nutrients in early development can influence long-term health and viability. However, little is known about the mechanisms of nutritional programming. The live-bearing fish Xiphophorus multilineatus has the potential to be a new model for understanding these mechanisms, given prior evidence of nutritional programming influencing behavior and juvenile growth rate. We tested the hypotheses that nutritional programming would influence behaviors involved in energy homeostasis as well gene expression in X. multilineatus.
Methods
We first examined the influence of both juvenile environment (varied in nutrition and density) and adult environment (varied in nutrition) on behaviors involved in energy acquisition and energy expenditure in adult male X. multilineatus. We also compared the behavioral responses across the genetically influenced size classes of males. Males stop growing at sexual maturity, and the size classes of can be identified based on phenotypes (adult size and pigment patterns). To study the molecular signatures of nutritional programming, we assembled a de novo transcriptome for X. multilineatus using RNA from brain, liver, skin, testis and gonad tissues, and used RNA-Seq to profile gene expression in the brains of males reared in low quality (reduced food, increased density) and high quality (increased food, decreased density) juvenile environments.
Results
We found that both the juvenile and adult environments influenced the energy intake behavior, while only the adult environment influenced energy expenditure. In addition, there were significant interactions between the genetically influenced size classes and the environments that influenced energy intake and energy expenditure, with males from one of the four size classes (Y-II) responding in the opposite direction as compared to the other males examined. When we compared the brains of males of the Y-II size class reared in a low quality juvenile environment to males from the same size class reared in high quality juvenile environment, 131 genes were differentially expressed, including metabolism and appetite master regulator agrp gene.
Discussion
Our study provides evidence for nutritional programming in X. multilineatus, with variation across size classes of males in how juvenile environment and adult diet influences behaviors involved in energy homeostasis. In addition, we provide the first transcriptome of X. multilineatus, and identify a group of candidate genes involved in nutritional programming.
The fish genus Xiphophorus consists of 26 species distributed along the eastern slopes of mountain ranges extending from northern Mexico to Belize and Nicaragua. We analyzed light-dependent repair of UV-induced DNA damage in at least two species from each of the four monophyletic Xiphophorus groups. We found that the northern platyfish had significantly reduced photoenzymatic repair compared to the other three groups, including the northern swordtails, southern platyfish and southern swordtails. All of the species of the platyfish, including the Marbled (meyeri), Northern (gordoni) and Monterrey Platyfish (couchianus) are the northernmost species in the genus and are the only three species in the genus that are currently found on the IUCN Red List of Threatened Species. Satellite data from the past 30 years (1979-2008) correlates greater increases in shorter wavelength UVB with higher latitudes within the Xiphophorus range. We suggest that, combined with other consequences of human population growth, anthropogenic deozonation resulting in a disproportionate increase in UVB in temperate latitudes may be a contributing factor in the decline and extirpation of the northern platyfish.This article is protected by copyright. All rights reserved.
Abstract The genus Xiphophorus has great potential to contribute to the study of vertebrate pigmentation and elucidating the relative influence of ecology, physiology, and behavior on evolution at the molecular level. More importantly, the association between pigmentation and a functional oncogene offers the potential to understand the evolution and maintenance of cancer-causing genetic elements. Using criteria laid out recently in the literature, I demonstrate the power of the Xiphophorus system for studying pigment evolution through integrative organismal biology. Using the most recent phylogeny, the phylogenetic distribution of several important pigmentation loci are reevaluated. I then review support for existing hypotheses of the functional importance of pigmentation. Finally, new observations and hypotheses regarding some of the characteristics of pigment patterns in natural populations and open questions and future directions in the study of the evolution of these traits are discussed.
Poeciliids are of particular interest to conservation biologists for a number of reasons. First, they have become excellent models for conservation biologists due to their small size and rapid generation time (Quattro & Vrijenhoek 1989; Leberg 1990, 1993). Second, many poeciliid studies have provided important insights on the rate of evolutionary diversification (e.g., Endler 1980; Reznick et al. 1990; Reznick et al. 1997; see also Pires et al., chapter 3; Breden & Lindholm, chapter 22; Grether & Kolluru, chapter 6; Johnson & Bagley, chap-ter 4; Schlupp & Riesch, chapter 5), the ultimate source of biodiversity. Third, poeciliids are vulnerable to the same anthropogenic factors driving the current extinction crisis (e.g., habitat loss, invasive species). Paradoxically, one of the greatest threats to poeciliids is the spread of heterospecifi c poeciliids (Minckley & Deacon 1968; Meffe 1985b; Minckley & Jensen 1985; Courtenay & Meffe 1989; Belk & Lydeard 1994). Thus, poeciliids have been extensively evaluated in the context of conservation biology (Johnson & Hubbs 1989; Leberg 1990, 1993; Stockwell et al. 1996; Stockwell & Weeks 1999). Johnson and Hubbs (1989) provided an overview of the conservation status of the poeciliids in the United States. Since that time, poeciliids have mirrored the decline of other freshwater fishes (see Jelks et al. 2008). Furthermore, our understanding of relationships among taxa has increased, allowing reevaluation of the systematics and distribution of poeciliid biodiversity (Rosen & Bailey 1963; Parenti 1981; Parenti & Rauchenberger 1989; Hrbek et al. 2007). Much applied research has focused on endangered poeciliids (e.g., Poeciliopsis spp.) as well as common and invasive poeciliids (e.g., Gambusia spp., Poecilia reticulata). In fact, many poeciliids have been used as model systems to examine questions central to the field of conservation biology. Here, we provide an overview of the poeciliids from the perspective of conservation biology. First, we review the geographic distribution of poeciliid biodiversity as well as associated threats. We then consider the impact of nonnative poeciliids on native species and ecosystems. We conclude by summarizing the role of selected poeciliid species as models for studies in the emerging fi eld of evolutionary conservation biology (as conceptualized by Ferrière et al. 2004).
Twenty-nine populations of Xiphophorus fishes representing nine species of northern swordtails, one southern swordtail and a platyfish were assayed electrophoretically for allozyme variation. Phylogenetic relationships were inferred using parsimony and likelihood analysis of gene frequency characters, as well as Fitch-Margoliash, minimum evolution and neighbor-joining analyses of genetic distances. The phylogenetic relationships among species that were well supported in all analyses included (1) monophyly of the northern swordtails, (2) the pygmaeus clade of Xiphophorus nigrensis, X. multilineatus, and X. pygmaeus, and (3) the clade of X. nigrensis and X. multilineatus. Of those species represented by more than one population, all analyses supported monophyly of X. montezumae and weakly supported monophyly of X. nezahualcoyotl and X. birchmanni. Only the distance analyses supported monophyly of X. cortezi, and the support was weak. Finally, all analyses supported a clade including X. nezahualcoyotl from the Río Tamesí drainage and some populations from the Río Pánuco drainage, that is, nonmonophyly of the set of populations from the Río Pánuco drainage. Previously published trees based on morphology, behavior and randomly amplified DNAs were generally congruent with the optimal trees for the allozyme data and were not rejected by those data; in contrast, trees based on DNA sequences were more incongruent with the optimal trees for the allozyme data and were rejected by those data.
The evolution of sexual signaling systems is influenced by natural and sexual selection acting on complex interactions among traits. Natural hybrid zones are ex- cellent systems for assessing fitness effects on sexual phenotypes. Most documented hybrid zones, however, show little variation in sexual signals. A hybrid zone between the swordtails Xiphophorus birchmanni and Xiphophorus malinche is characterized by numerous recombinants for male sexual traits. Analyses of geographic variation in morphological and isozyme traits in the Rio Calnali, Hidalgo, Mexico, reveal an upstream-to-downstream gradient from X. malinche -t oX. birchmanni-type traits. A second hybrid zone, likely isolated from the R. Calnali, occurs in the nearby Arroyo Pochutla. Although the presumed female preference for swords predicts the intro- gression of swords from X. malinche-like populations into hybrid populations, the opposite pattern was observed. Swords are reduced in populations otherwise char- acterized by X. malinche traits. Sexually dimorphic traits were poorly correlated with- in individuals, indicating that sexual selection does not act against recombinant phe- notypes. Hybrid males also exhibit trait values outside the range of parental varia- tion. These patterns are consistent with predictions that females are permissive, preferring generally conspicuous males without attending to specific features.
The swordtail, Xiphophorus clemenciae (Poeciliidae), has been considered a species of special concern because of its apparent limited range. Although described in 1959, it is officially still known only from three locations in the Rio Coatzacoalcos basin, Mexico. Zoogeographic studies have now shown that this species is widespread and abundant but restricted to the uplands of the Rio Coatzacolacos basin where it replaces in many areas the common swordtail, X. helleri. Two new swordtail taxa, X. mixei and X. monticolus, are described from headwater streams of the Rio Jaltepec, a major Rio Coatzacoalcos tributary, Oaxaca, Mexico. The new forms are sympatric in part of their range and replace both X. clemenciae and X. helleri. Morphometric and molecular analyses revealed that X. clemenciae and the two new species constitute a monophyletic clade that exhibits a closer evolutionary affinity to the “northern” swordtails and the “platyfish” group of the genus rather than to X. helleri and the other “southern” swordtails. The evolutionary relationships of these taxa are discussed.
The Mexican freshwater fish that are at risk orextinct are listed by family, state,basin/region, and causes of risk. Of the 506known species, 168 are at some level of risk,and 25 are believed to be extinct. States withthe most reports are: Chihuahua (46); Coahuila(35); Nuevo Len (20); Sonora (19); Durango(18); and Tamaulipas (15). With the exceptionof Sonora, these states are largely located inthe Ro Bravo region, and are all arid orsemiarid. Most extinctions have occurred inNuevo Len (8) and Coahuila (7). The maincauses of risk reported are: habitat reductionor alteration (86); water depletion (83);presence of exotics (76); small or decliningpopulation (73); and small habitat (57). Allof which result in very local endemism. Thesecauses of risk develop easily when thedistribution is small. The critical factor isaridity, which is associated withdesertization. Water conflicts were to beexpected, and the result is resource nonsustainability.
The northern and central part of Veracruz, Mxico, as herein delimited, is inhabited by 121 species of fish, in 83 genera and 47 families. Four species and 3 families are Nearctic, 28 species and 6 families are Neotropical, and 1 species is circumtropical. Ecologically, 8 species and 4 families are primary, 25 species and 6 families are secondary, 30 families are peripheral, and 7 families are marine, many of them marine accidentals. A mountainous area near Punta del Morro almost reaches the sea, and acts as a filter for primary and secondary species. Its importance had not been described in detail. From the north, the following species range to Punta del Morro or less:astyanax mexicanus, Dionda ipni, Ictalurus cf.furcatus, I. cf.punctatus, Lucania parva, Cyprinodon variegatus, Gambusia affinis, G. vittata, Heterandria sp.,Poecilia formosa, P. latipinna, Xiphophorus birchmani, X. variatus, Cichlasoma cyanoguttatum, C. labridens andC. sp.. From the south, the species that approach or reach Punta del Morro are:Astyanax aeneus, Hyphessobrycon compressus, Rhamdia guatemalensis, R. laticauda, Rivulus cf.robustus, Belonesox belizanus, Poeciliopsis gracilis, Xiphophorus andersi, X. maculatus, andOphisternon aenigmaticum. Eight species including one peripheral are present on both sides and constitute short penetrations across the barrier. At least nine species represent introductions, purposeful or accidental.
Gambusia regani andG. panuco are nominal species of the same form: by first reviser,G. regani is regarded as senior synonym.
Arbitrarily primed PCR (AP-PCR or RAPD) is a technique for producing species-specific DNA fingerprints. We tested the utility of AP-PCR as a source of phylogenetically informative characters in three separate experiments, using fishes of the genus Xiphophorus. We chose Xiphophorus as a standard of comparison, because evolutionary relationships within the genus have been studied repeatedly using a variety of techniques. We compared our results to a "classical" phylogenetic hypothesis synthesized from studies using morphological, pigmentation, and allozyme characters, and to a recent conflicting hypothesis constructed from DNA sequence data. The sequence-based hypothesis places the southern swordtail Xiphophorus clemenciae squarely within the platyfish, whereas the classical hypothesis separates the two groups. In addition, the two hypotheses differ in their clustering of species of northern swordtails. Our findings are in close accord with the classical hypothesis. Our results allow the strongest phylogenetic hypothesis yet for Xiphophorus and demonstrate the utility of AP-PCR for studying species relationships within vertebrate genera.
It has been hypothesized that morphological diversity within clades can be generated by simple alterations of shared developmental programs. However, few studies have examined changes in heterochrony, the rate and timing of developmental events, in an explicitly phylogenetic context. We studied how developmental patterns have changed phylogenetically in the northern swordtail clade of Xiphophorus . We reared individuals of an outgroup and seven of nine species in the clade and followed their development for ∼300 days. For each individual, we used nonlinear regression to estimate three growth parameters: growth rate, adult body size, and age of cessation of growth. We estimated sword growth rates in males by linear regression. We then used the means of these growth parameters to construct standard growth curves for each species and to study growth patterns in a phylogenetic context. A combined phylogeny was constructed from both phenotypic and DNA sequence data. The phenotypic data set, compiled from the literature, consisted of 86 morphological, pigmentation, behavioral, and random amplified polymorphic DNA characters, many of which had not been used before for phylogenetic analysis. DNA sequence data from three genes for a total of 1284 bases were also obtained from the literature and included in the analysis. Relationships between growth parameters were examined by phylogenetically independent contrasts in relation to seven different phylogenies based on the most-parsimonious trees generated from the phenotypic, DNA sequence, and combined data sets; this allowed us to identify relationships between variables that were not sensitive to ambiguities in Xiphophorus phylogeny. Our analysis revealed statistically significant correlations between female body size and male body size, and between female growth rate and male sword growth rate, for all seven phylogenies. Marginally statistically significant relationships were also identified between female body size and female growth rate, and between female growth rate and male body size. We relate these relationships to what is known about the ecology, genetics, and behavior of Xiphophorus to better understand the evolution of growth patterns of both the body as a whole and the sword in particular. The relationship of these data to the evolution of swords is discussed.
DARWIN believed that sexual selection accounts for the evolution of exaggerated male ornaments, such as the sword-like caudal fin extensions of male fishes of the genus Xiphophorus, that appear detrimental to survival. Swordtails continue to feature prominently in empirical work and theories of sexual selection; the pre-existing bias hypothesis has been offered as an explanation for the evolution of swords in these fishes. Based upon a largely morphological phylogeny, this hypothesis suggests that female preference to mate with sworded males arose in ancestrally swordless species, thus pre-dating the origin of the sword itself and directly driving its evolution. Here we present a molecular phylogeny (based on mitochondria! and nuclear DNA sequences) of Xiphophorus which differs from the traditional one: it indicates that the sword originated and was lost repeatedly. Our phylogeny suggests that the ancestor of the genus is more likely to have possessed a sword than not, thus questioning the applicability of the pre-existing bias hypothesis as an explanation for the evolution of this sexually selected trait.
Fishes of the genus Xiphophorus (platyfishes and swordtails) are small, internally fertilizing, livebearing, and derived from freshwater habitats in Mexico,
Guatemala, Belize, and Honduras. Scientists have used these fishes in cancer research studies for more than 70 yr. The genus
is presently composed of 22 species that are quite divergent in their external morphology. Most cancer studies using Xiphophorus use hybrids, which can be easily produced by artificial insemination. Phenotypic traits, such as macromelanophore pigment
patterns, are often drastically altered as a result of lack of gene regulation within hybrid fishes. These fish can develop
large exophytic melanomas as a result of upregulated expression of these pigment patterns. Because backcross hybrid fish are
susceptible to the development of melanoma and other neoplasms, they can be subjected to potentially deleterious chemical
and physical agents. It is thus possible to use gene mapping and cloning methodologies to identify and characterize oncogenes
and tumor suppressors implicated in spontaneous or induced neoplasia. This article reviews the history of cancer research
using Xiphophorus and recent developments regarding DNA repair capabilities, mapping, and cloning of candidate genes involved in neoplastic
phenotypes. The particular genetic complexity of melanoma in these fishes is analyzed and reviewed.
Xiphophorus meyeri n. sp. is described as an endemic to Muzquiz, Coahuila, Mexico. It appears to be the northernmost species of the genus. The new species is related to X. couchianus and X. gordoni, but differs morphologically from those by dorsal fin ray number, by the expression of some gonopodial features and most markedly by the appearance of macromelanophores or tr-melanophores.
Xiphophorus gordoni, the third new species of this genus to be made known since 1959, is described and figured from the arid, semi-isolated Cuatro Ciénegas basin of central Coahuila, México. It is based on meristic, morphometric, and osteological data-and observations of living material in the field and laboratory-involving more than 200 individuals. The new species is related to X. couchianus, from the vicinity of Monterrey, and to X. variatus xiphidium, from the basin of the Río Soto la Marina-possibly more intimately to the latter. X. gordoni is known only from a single body of water, of which a photograph is presented. It is one of a number of fishes endemic to the Cuatro Ciénegas basin.
Small isolated populations are generally considered to have become homozygous at many gene loci. However, in nearly all cases it is quite difficult to demonstrate experimentally such a decrease in the genetic variability, since suitable genetic markers are usually absent from most species. An attempt has been made to determine by means of tissue transplantation whether small populations are "naturally" inbred. Since the genetic requirements for graft survival are very stringent and since many genes are involved in determining the fate of a graft, the survival of a transplant can be taken as an excellent indication that host and donor posses similar, if not identical, genotypes. Fin transplants were exchanged among the offspring (intrasib grafts) of 15 wild-caught platyfish, Xiphophorus couchianus, collected in 8 different springs or pools. A high percentage (25-69) of the grafts were accepted permanently among sibs whose parents had been collected in springs where the adult population numbered only a few dozen or less. The percentage of successful intrasib grafts declined sharply when the parents came from large populations.
Data are presented from studies on heritable melanoma formation in certain hybrids of Xiphophorus fishes. In the best investigated case, benign and malignant melanomas were caused primarily by hemizygous and homozygous loss, respectively, of a tumor suppressor gene that promoted final pigment cell differentiation, although additional genetic events may be of importance. In other melanoma cases, different genes, most of them presumably also tumor suppressor genes, have been implicated in the development of melanomas and may interact at other levels of pigment cell development, such as commitment, migration, or homing of pigment cell precursors. Importantly, genetic factors have been identified that do not allow melanoma formation despite the loss of tumor suppressor genes. Taken together, these findings stress the complexity of tumorigenesis and the heterogeneity of pathways that lead to cancers of the same histological type.
The onset of sexual maturity in fishes of the genus Xiphophorus depends upon a multiple allelic series at the P locus. X. helleri of genotype PhPh initiate sexual maturity at 10.4 weeks and X. maculatus of genotypes P3P3 or P4P4 at 8.7 or 20 weeks, respectively. Hybrids between the two species of genotype PhPh and possessing some autosomes of X. maculatus in homozygous condition exhibit precocious maturation at 5.3 weeks and are correspondingly small. PhPh hybrids of the backcross generations to X. helleri mature at the same age and size as X. helleri. The P3Ph and P4Ph backcross hybrids, however, exhibit greatly delayed maturation, and the delay becomes more severe as the number of X. helleri autosomes increases. The phenotype results from the interaction of the P locus with a system of regulatory genes. Both the P factors and the regulatory systems are species-specific.
Synthetic luteinizing hormone-releasing hormone (LH-RH) was effective in inducing proliferation and functional activity in the gonadotrops and ultimately sexual maturity in all genotypes, including PhP3 hybrids. Whereas the response of different genotypes to gonadotropin was essentially the same, their response to LH-RH was significantly different. A positive correlation exists between the rapidity of the response to LH-RH and the genetically determined age at which spontaneous maturation takes place. The maturity induced by LH-RH was not permanent. After cessation of LH-RH administration the gonadotrops became inactive and regressed. These observations suggest that the P locus is not concerned with the production of LH-RH. Rather, the P locus appears to control the physiological function or the fate of LH-RH.
The inheritance of two Y-linked colour genes of the Poecillid Xiphophorus pygmaeus nigrensis has been described. Males with the gene Fl (=flavus) posess a yellow colouration of the body whereas those with the gene Vfl (= venter flavus) only at the basis of the anal ray. These patterns depend on G-xanthophores, yellow pigment cells with colourless granulation (Öktay).
Cross-breedings with X. maculatus and X. milleri cause an intensification of the patterns by increase of the number of the G-xanthophores and their transformation into xanthoerythrophores (colour cells with red granulation). Backcross males with X. maculatus inheriting Fl or Vfl show a red colouration of the whole body.
These results may be explained by the theory of Kosswig, Gordon and Öktay of polygenic modifier systems. Their function is to regulate the number, the intensity and the grade of pigment cells, while the colour gene activates this process of establishing the pattern and determines the center of it. Modifiers in X. pygmaeus nigrensis changing G-xanthophores into xanthoerythrophores are below a genetic level or absent.
Zusammenfassung Nachweis von genetisch bedingten Melanomen bei Wildfängen der reinen Art des ZahnkarpfenXiphophorus variatus, wobei es sich eindeutig nicht um einen Artbastard handelt.
Hybrids between distinct Xiphophorus species have been utilized for over 70 years to study melanoma and other neoplasms that can develop spontaneously in hybrid offspring. Genetic linkage mapping has proven to be important in delineating genomic areas that harbor oncogenes and tumor suppressors. Within this report, two parallel backcrosses have been utilized to generate a genetic linkage map for Xiphophorus fishes. Isozyme/allozyme, RFLP and PCR-based mapping techniques, including AP-PCR/RAPDs and microsatellite loci were utilized. The derived linkage map provides a total of 403 mapped polymorphisms distributed among 24 linkage groups, representative of 24 acro- and telocentric chromosome pairs. Genomic coverage is approximately one marker per 5.8 cM. Detailed genotypic analysis of the utilized hybrids revealed two areas of the genome that show significant segregation distortion. Loci within the linkage group harboring the sex determining locus (LG 24) and an autosomal linkage group (LG 21) show highly significant deviations from Mendelian expectations. This phenomenon is not present in a hybrid cross that utilizes a different backcross hybrid progenitor species. The derived map with sequence-tagged markers provides a framework for physical map generation, large-scale genomic sequencing and will further enable cross-genome comparisons of vertebrate genomes.
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